Method for preparing anhydrous calcium sulfate from ardealite and synchronously removing phosphorus and fluorine

Abstract

The invention provides a method for preparing anhydrous calcium sulfate from ardealite and synchronously removing phosphorus and fluorine, which comprises the following steps: mixing ardealite and a sulfuric acid solution for leaching reaction, filtering to respectively obtain filter residue and filtrate, and drying the filter residue to obtain anhydrous calcium sulfate; adding ardealite into the filtrate for leaching reaction, and filtering to obtain anhydrous calcium sulfate and filtrate b; repeatedly circulating the filtrate b for multiple times to perform operations of phosphogypsum addition and leaching reaction, collecting to obtain enriched filtrate, adding rare earth oxide and hydrogen peroxide, stirring to perform impurity removal reaction, and performing solid-liquid separation to respectively obtain precipitate rare earth phosphate and purified filtrate; carrying out recovery treatment on the precipitate rare earth phosphate; adding the purified filtrate into the filtrate, and carrying out circular reaction; volatilized fluorine-containing steam is collected in the stirring process, condensed and collected. The method provided by the invention can realize synergistic removal of phosphorus and fluorine in phosphogypsum.

Description

Technical Field

[0001] This invention belongs to the field of resource utilization technology of phosphogypsum, and specifically relates to a method for preparing anhydrous calcium sulfate from phosphogypsum while simultaneously removing phosphorus and fluoride. Background Technology

[0002] Phosphogypsum is a solid byproduct emitted during the wet-process phosphoric acid production, and its production volume is enormous. Statistics show that my country's annual phosphogypsum emissions exceed 80 million tons, with accumulated stockpiles exceeding 600 million tons. These large stockpiles not only occupy land resources but also easily cause groundwater pollution and environmental safety hazards, urgently requiring efficient and green resource utilization methods.

[0003] Anhydrous calcium sulfate, as an important raw material in chemical and building materials, has wide applications in cement retarders, specialty building materials, inorganic fillers, and sulfuric acid recycling processes, possessing a utilization value far exceeding that of ordinary gypsum. Realizing the high-value conversion of phosphogypsum into anhydrous calcium sulfate not only helps to dispose of the massive stockpiles of phosphogypsum but also brings considerable economic benefits. However, phosphogypsum often contains impurities such as phosphorus and fluorine ions. These impurities not only affect the purity and performance of anhydrous calcium sulfate products but also severely interfere with the recycling of acid leaching agents, leading to low resource utilization efficiency.

[0004] Currently, the mainstream methods for the resource utilization of phosphogypsum mostly employ strong acid systems such as sulfuric acid. However, this method has significant shortcomings in removing impurities such as phosphorus and fluorine, often requiring multi-stage neutralization or auxiliary precipitation processes. These processes are complex, consume large amounts of reagents, and are difficult to treat as byproduct wastewater, presenting significant economic and environmental challenges. These deficiencies severely restrict the promotion and application of phosphogypsum in the preparation of high-purity anhydrous calcium sulfate.

[0005] Rare earth oxides dissolve in acidic systems to form rare earth ions, which have a strong affinity for phosphate ions, resulting in the formation of rare earth phosphate precipitates with extremely low solubility, thus achieving selective phosphorus removal. The unique advantage of this process is that phosphorus can be directly separated from the strong acid system without disrupting the original leaching agent environment, allowing for the recycling of the leaching agent and avoiding additional neutralization or extraction steps. Simultaneously, rare earth phosphates generally have a higher value than rare earth oxides, achieving both phosphorus extraction and high-value regeneration of rare earth oxides. Furthermore, fluoride ions dissolved in the leachate are volatile and can further purify the system, contributing to the acquisition of high-purity anhydrous calcium sulfate. Summary of the Invention

[0006] The technical problem this invention aims to solve is to address the shortcomings of the prior art by providing a method for the simultaneous removal of phosphorus and fluoride from anhydrous calcium sulfate prepared from phosphogypsum. This method introduces rare earth oxides into a strong sulfuric acid system, causing them to dissolve and release rare earth ions. These ions react with phosphate ions in the solution to form rare earth phosphate precipitates with extremely low solubility, achieving selective removal of phosphorus under strong acid conditions. Simultaneously, regarding fluoride, this invention utilizes a volatile treatment process after the leaching filtrate is recycled and enriched, allowing fluoride to be enriched and separated as volatile fluorides, thereby achieving synergistic removal of both phosphorus and fluoride.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for simultaneously removing phosphorus and fluoride from anhydrous calcium sulfate prepared from phosphogypsum, the method being as follows: S1. Mix phosphogypsum and sulfuric acid solution, and stir at a temperature of 25℃~100℃ for 30min~150min to obtain the reaction product. S2. After filtering the reaction product obtained in S1, filter residue and filtrate are obtained respectively. After drying the filter residue, anhydrous calcium sulfate is obtained. S3. Add phosphogypsum to the filtrate obtained in S2. Stir and carry out the leaching reaction for 30 min to 150 min at a temperature of 25℃ to 100℃. Filter to obtain anhydrous calcium sulfate and filtrate b. Repeat the addition of phosphogypsum and leaching reaction of the obtained filtrate b in this step. Repeat the cycle multiple times and collect the enriched filtrate. S4. At a temperature of 40℃~90℃, rare earth oxides are added to the enriched filtrate obtained in S3 and stirred for 30min~120min. Then hydrogen peroxide is added and stirred for 30min~120min to carry out the impurity removal reaction. After solid-liquid separation, the precipitate rare earth phosphate and the purified filtrate are obtained respectively. S5. Recover and process the rare earth phosphate precipitate obtained in S4. S6. Add the purified filtrate obtained in S4 to the filtrate obtained in S2 and cycle the reaction. S7. During the stirring process in S4, collect the volatilized fluorine vapors, which are then condensed and collected as fluorides.

[0008] Preferably, the sulfuric acid solution in S1 is an aqueous sulfuric acid solution with a mass fraction of 5% to 30%.

[0009] Preferably, the mass ratio of phosphogypsum to sulfuric acid solution in S1 is 1:(1-10); the mass ratio of phosphogypsum to filtrate in S3 is 1:(1-10).

[0010] Preferably, the rare earth oxide in S4 is one or more of cerium oxide, lanthanum oxide, neodymium oxide, and dysprosium oxide.

[0011] Preferably, the molar ratio of the rare earth oxide and hydrogen peroxide in S4 is 1:(1-5); the molar ratio of the rare earth oxide and phosphorus in the enriched filtrate is 0.5-2.0.

[0012] Preferably, the number of cycles for S3 is 10 to 50.

[0013] This invention is applicable not only to ordinary phosphogypsum, but also to various gypsum raw materials such as titanium gypsum, fluorogypsum, desulfurized gypsum, and hemihydrate gypsum.

[0014] Compared with the prior art, the present invention has the following advantages: 1. This invention addresses the technical bottlenecks of phosphorus and fluorine impurities commonly found in phosphogypsum, which lead to low purity of anhydrous calcium sulfate products, reduced leaching agent recycling performance, and the difficulty of efficient removal using traditional acid methods. This invention introduces rare earth oxides into a strong sulfuric acid system (sulfuric acid solution), causing them to dissolve and release rare earth ions. These ions react with phosphate ions in the solution to form rare earth phosphate precipitates with extremely low solubility, achieving selective removal of phosphorus under strong acid conditions. Simultaneously, for fluorine, this invention utilizes a volatilization process after the leaching filtrate is recycled and enriched, allowing fluorine to be concentrated and separated as volatile fluorides, thus achieving synergistic removal of phosphorus and fluorine. This invention achieves efficient separation of phosphorus and fluorine under highly acidic conditions, avoiding the complex processes of additional pH adjustment or multi-step precipitation and extraction required in traditional methods. This ensures the recycling and long-term stability of the leaching agent (filtrate). Furthermore, the obtained anhydrous calcium sulfate has high purity and can be directly applied in high-value fields such as chemical, building materials, and functional materials.

[0015] The present invention will be further described in detail below with reference to the embodiments. Detailed Implementation

[0016] Example 1 In this embodiment, the phosphogypsum is the waste residue phosphogypsum from wet-process phosphoric acid.

[0017] The method for preparing anhydrous calcium sulfate from phosphogypsum and simultaneously removing phosphorus and fluoride in this embodiment is as follows: S1. Mix 500g of phosphogypsum and 4kg of 15% sulfuric acid aqueous solution, stir at 60℃, and carry out leaching reaction for 90min to obtain reaction product. S2. After filtering the reaction product obtained in S1, filter residue and filtrate are obtained respectively. After drying the filter residue, anhydrous calcium sulfate is obtained. S3. Add phosphogypsum to the filtrate obtained in S2, stir at 80℃, and carry out the leaching reaction for 60 minutes to dehydrate calcium sulfate dihydrate to produce anhydrous calcium sulfate. After filtration, anhydrous calcium sulfate and filtrate b are obtained respectively. The leached filtrate b is used for the leaching of new phosphogypsum. That is, the filtrate b obtained is subjected to the same steps of adding phosphogypsum and leaching reaction, and the cycle is repeated 20 times to enrich phosphorus and fluoride ions. The enriched filtrate is then collected. The mass ratio of phosphogypsum to filtrate is 1:8. The enriched filtrate was tested by ICP-OES and found to contain 2532 ppm of phosphorus and 632 ppm of fluorine. S4. At a temperature of 90℃, rare earth oxide CeO2 is added to the enriched filtrate obtained in S3 and stirred for 30 min. Then hydrogen peroxide is added and stirred for 30 min to carry out the impurity removal reaction. The rare earth oxide reacts with the phosphate in the enriched filtrate to form a low-solubility rare earth phosphate precipitate. After solid-liquid separation, the precipitate rare earth phosphate and the purified filtrate are obtained respectively. The phosphorus content in the filtrate after purification of the liquid phase was reduced, with a phosphorus removal rate of 82%. The molar ratio of the rare earth oxide to hydrogen peroxide is 1:3; the molar ratio of the rare earth oxide to phosphorus in the enriched filtrate is 2.0. S5. Recover and process the rare earth phosphate precipitate obtained in S4. S6. Add the purified filtrate obtained in S4 to the filtrate obtained in S2 and cycle the reaction. S7. During the stirring process in S4, fluorides in water vapor, i.e., volatile fluorine-containing vapors, are collected. After condensation, they are converted into fluorides for collection, realizing the separation and enrichment of fluorine. The fluorine volatilization removal rate is 80%.

[0018] The anhydrous calcium sulfate separated in this embodiment has a purity of 91% and can be directly applied to high-value fields such as chemical industry, building materials and functional materials.

[0019] Example 2 In this embodiment, the phosphogypsum is the waste residue phosphogypsum from wet-process phosphoric acid.

[0020] The method for preparing anhydrous calcium sulfate from phosphogypsum and simultaneously removing phosphorus and fluoride in this embodiment is as follows: S1. Mix 600g of phosphogypsum and 3kg of 18% sulfuric acid aqueous solution, stir at 65℃, and carry out leaching reaction for 100min to obtain reaction product. S2. After filtering the reaction product obtained in S1, filter residue and filtrate are obtained respectively. After drying the filter residue, anhydrous calcium sulfate is obtained. S3. Add phosphogypsum to the filtrate obtained in S2, stir at a temperature of 80℃, and carry out a leaching reaction for 100 minutes to dehydrate calcium sulfate dihydrate to produce anhydrous calcium sulfate. After filtration, anhydrous calcium sulfate and filtrate b are obtained respectively. The leached filtrate b is used for the leaching of new phosphogypsum. The process of adding phosphogypsum and leaching reaction is repeated in the filtrate b in this step. This cycle is repeated 12 times to enrich phosphorus and fluoride ions. The enriched filtrate is then collected. The enriched filtrate was tested by ICP-OES and found to contain 3000 ppm of phosphorus and 550 ppm of fluorine. S4. At a temperature of 80℃, rare earth oxide cerium oxide (CeO2) is added to the enriched filtrate obtained in S3 and stirred for 60 min. Then hydrogen peroxide is added and stirred for 30 min to carry out the impurity removal reaction. The rare earth oxide reacts with the phosphate in the enriched filtrate to form a low-solubility rare earth phosphate precipitate. After solid-liquid separation, the precipitate rare earth phosphate and the purified filtrate are obtained respectively. The phosphorus content in the filtrate after purification of the liquid phase was reduced, with a phosphorus removal rate of 90%. The molar ratio of the rare earth oxide to hydrogen peroxide is 1:1; the molar ratio of the rare earth oxide to phosphorus in the enriched filtrate is 1.5. S5. Recover and process the rare earth phosphate precipitate obtained in S4. S6. Add the purified filtrate obtained in S4 to the filtrate obtained in S2 and cycle the reaction. S7. During the stirring process in S4, fluorides in water vapor, i.e., volatile fluorine-containing vapors, are collected. After condensation, they are converted into fluorides for collection, realizing the separation and enrichment of fluorine. The fluorine volatilization removal rate is 85%.

[0021] The anhydrous calcium sulfate isolated in this embodiment has a purity of 93%.

[0022] Example 3 In this embodiment, the phosphogypsum is the waste residue phosphogypsum from wet-process phosphoric acid.

[0023] The method for preparing anhydrous calcium sulfate from phosphogypsum and simultaneously removing phosphorus and fluoride in this embodiment is as follows: S1. Mix 500g of phosphogypsum and 5kg of 30% sulfuric acid aqueous solution, stir at 25℃, and carry out leaching reaction for 150min to obtain reaction product. S2. After filtering the reaction product obtained in S1, filter residue and filtrate are obtained respectively. After drying the filter residue, anhydrous calcium sulfate is obtained. S3. Add phosphogypsum to the filtrate obtained in S2, stir at a temperature of 25°C, and carry out a leaching reaction for 150 minutes to dehydrate calcium sulfate dihydrate to produce anhydrous calcium sulfate. After filtration, anhydrous calcium sulfate and filtrate b are obtained respectively. The leached filtrate b is used for the leaching of new phosphogypsum. That is, the filtrate b obtained is subjected to the same steps of adding phosphogypsum and leaching reaction, and the cycle is repeated 50 times to enrich phosphorus and fluoride ions. The enriched filtrate is then collected. The mass ratio of phosphogypsum to filtrate is 1:10. The enriched filtrate was tested by ICP-OES and found to contain 6200 ppm of phosphorus and 1000 ppm of fluorine. S4. At a temperature of 40℃, rare earth oxide cerium oxide (La2O3) was added to the enriched filtrate obtained in S3 and stirred for 120 min. Then hydrogen peroxide was added and stirred for 120 min to carry out the impurity removal reaction. The rare earth oxide reacted with the phosphate in the enriched filtrate to form a low-solubility rare earth phosphate precipitate. After solid-liquid separation, the precipitate rare earth phosphate and the purified filtrate were obtained respectively. The phosphorus content in the filtrate after purification of the liquid phase was reduced, with a phosphorus removal rate of 90%. The molar ratio of the rare earth oxide to hydrogen peroxide is 1:5; the molar ratio of the rare earth oxide to phosphorus in the enriched filtrate is 0.5. S5. Recover and process the rare earth phosphate precipitate obtained in S4. S6. Add the purified filtrate obtained in S4 to the filtrate obtained in S2 and cycle the reaction. S7. During the stirring process in S4, fluorides in water vapor, i.e., volatile fluorine-containing vapors, are collected. After condensation, they are converted into fluorides for collection, realizing the separation and enrichment of fluorine. The fluorine volatilization removal rate is 70%.

[0024] The anhydrous calcium sulfate isolated in this example has a purity of 94%.

[0025] Example 4 In this embodiment, the phosphogypsum is the waste residue phosphogypsum from wet-process phosphoric acid.

[0026] The method for preparing anhydrous calcium sulfate from phosphogypsum and simultaneously removing phosphorus and fluoride in this embodiment is as follows: S1. Mix 1000g of phosphogypsum and 1000g of 5% sulfuric acid aqueous solution, stir at 100℃, and carry out leaching reaction for 30 minutes to obtain reaction product. S2. After filtering the reaction product obtained in S1, filter residue and filtrate are obtained respectively. After drying the filter residue, anhydrous calcium sulfate is obtained. S3. Add phosphogypsum to the filtrate obtained in S2, stir at 100℃, and carry out the leaching reaction for 30 minutes to dehydrate calcium sulfate dihydrate to produce anhydrous calcium sulfate. After filtration, anhydrous calcium sulfate and filtrate b are obtained respectively. The leached filtrate b is used for the leaching of new phosphogypsum. That is, the filtrate b obtained is subjected to the same steps of adding phosphogypsum and leaching reaction, and the cycle is repeated 10 times to enrich phosphorus and fluoride ions. The enriched filtrate is then collected. The mass ratio of phosphogypsum to filtrate is 1:1. The enriched filtrate was tested by ICP-OES and found to contain 1012 ppm of phosphorus and 305 ppm of fluorine. S4. At a temperature of 70℃, neodymium oxide (Nd2O3) was added to the enriched filtrate obtained in S3 and stirred for 40 min. Then hydrogen peroxide was added and stirred for 50 min to carry out the impurity removal reaction. The rare earth oxide reacted with the phosphate in the enriched filtrate to form a low-solubility rare earth phosphate precipitate. After solid-liquid separation, the precipitate rare earth phosphate and the purified filtrate were obtained respectively. The phosphorus content in the filtrate after purification of the liquid phase was reduced, with a phosphorus removal rate of 90%. The rare earth oxide is one or more of cerium oxide, lanthanum oxide, neodymium oxide, and dysprosium oxide; The molar ratio of the rare earth oxide to hydrogen peroxide is 1:3; the molar ratio of the rare earth oxide to phosphorus in the enriched filtrate is 1.2. S5. Recover and process the rare earth phosphate precipitate obtained in S4. S6. Add the purified filtrate obtained in S4 to the filtrate obtained in S2 and cycle the reaction. S7. During the stirring process in S4, fluorides in water vapor, i.e., volatile fluorine-containing vapors, are collected. After condensation, they are converted into fluorides for collection, realizing the separation and enrichment of fluorine. The fluorine volatilization removal rate is 80%.

[0027] The anhydrous calcium sulfate isolated in this embodiment has a purity of 90%.

[0028] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for simultaneously removing phosphorus and fluoride during the preparation of anhydrous calcium sulfate from phosphogypsum, characterized in that, The method is as follows: S1. Mix phosphogypsum and sulfuric acid solution, and stir at a temperature of 25℃~100℃ for 30min~150min to obtain the reaction product. S2. After filtering the reaction product obtained in S1, filter residue and filtrate are obtained respectively. After drying the filter residue, anhydrous calcium sulfate is obtained. S3. Add phosphogypsum to the filtrate obtained in S2. Stir and carry out the leaching reaction for 30 min to 150 min at a temperature of 25℃ to 100℃. Filter to obtain anhydrous calcium sulfate and filtrate b. Repeat the addition of phosphogypsum and leaching reaction of the obtained filtrate b in this step. Repeat the cycle multiple times and collect the enriched filtrate. S4. At a temperature of 40℃~90℃, rare earth oxides are added to the enriched filtrate obtained in S3 and stirred for 30min~120min. Then hydrogen peroxide is added and stirred for 30min~120min to carry out the impurity removal reaction. After solid-liquid separation, the precipitate rare earth phosphate and the purified filtrate are obtained respectively. S5. Recover and process the rare earth phosphate precipitate obtained in S4. S6. Add the purified filtrate obtained in S4 to the filtrate obtained in S2 and cycle the reaction. S7. During the stirring process in S4, collect the volatilized fluorine vapors, which are then condensed and collected as fluorides.

2. The method for preparing anhydrous calcium sulfate from phosphogypsum with simultaneous phosphorus and fluoride removal according to claim 1, characterized in that, The sulfuric acid solution mentioned in S1 is an aqueous solution of sulfuric acid with a mass fraction of 5% to 30%.

3. The method for preparing anhydrous calcium sulfate from phosphogypsum with simultaneous phosphorus and fluoride removal according to claim 1, characterized in that, The mass ratio of phosphogypsum to sulfuric acid solution in S1 is 1:(1-10); the mass ratio of phosphogypsum to filtrate in S3 is 1:(1-10).

4. The method for preparing anhydrous calcium sulfate from phosphogypsum with simultaneous phosphorus and fluoride removal according to claim 1, characterized in that, The rare earth oxides mentioned in S4 are one or more of cerium oxide, lanthanum oxide, neodymium oxide, and dysprosium oxide.

5. The method for preparing anhydrous calcium sulfate from phosphogypsum with simultaneous phosphorus and fluoride removal according to claim 1, characterized in that, The molar ratio of the rare earth oxide and hydrogen peroxide in S4 is 1:(1-5); the molar ratio of the rare earth oxide and phosphorus in the enriched filtrate is 0.5-2.

0.

6. The method for preparing anhydrous calcium sulfate from phosphogypsum with simultaneous phosphorus and fluoride removal according to claim 1, characterized in that, The number of cycles for S3 ranges from 10 to 50.