Method for directional generation of anhydrous calcium sulfate in the process of impurity removal from phosphogypsum wet leachate

Abstract

The application provides a method for generating anhydrous calcium sulfate in a phosphogypsum wet leaching solution impurity removal process, which comprises the following steps: oxidizing an acid leaching solution in a phosphogypsum wet crystallization process with an oxidizing agent at an addition rate of 0.1-2.0 mL / min, adding a pH adjuster at 40-80 DEG C to the acid leaching solution after oxidation until the pH value is 3.0-4.0, stirring, adding a combined additive composed of a nucleation control agent and a polyhydric alcohol water structure control agent to the acid leaching solution after pH adjustment, stirring, reaction and aging, and drying the precipitated product to obtain anhydrous calcium sulfate. The application introduces an oxidizing agent and a composite additive into the acid leaching solution purification process respectively, realizes efficient removal of iron and related impurity elements, synchronously controls the crystal phase transformation behavior of calcium sulfate, promotes the directional generation of anhydrous calcium sulfate, realizes the preparation of anhydrous calcium sulfate in the whole industry chain of the wet crystallization of large solid waste phosphogypsum, and provides technical support for the high-value utilization of phosphogypsum.

Description

Technical Field

[0001] This invention belongs to the field of resource utilization technology of phosphogypsum, specifically relating to a method for the directional generation of anhydrous calcium sulfate during the impurity removal process of phosphogypsum wet leachate. Background Technology

[0002] Phosphoric acid is a crucial raw material for the phosphate chemical industry, widely used in phosphate fertilizer production, fine chemicals, food, and pharmaceuticals. As a core raw material in the phosphate fertilizer industry, it plays a vital role in national agricultural production and food security. The wet-process phosphoric acid production mainly involves the decomposition of fluorapatite using sulfuric acid, nitric acid, and hydrochloric acid. The sulfuric acid process is widely used due to its mature technology, adaptability to raw materials, and low production costs. However, on average, producing one ton of phosphoric acid generates approximately 4.5 to 5.5 tons of phosphogypsum, an industrial byproduct. Statistics show that my country's annual phosphogypsum production is approximately 75 million tons, with a comprehensive utilization rate of only 45%, resulting in a cumulative stockpile exceeding 830 million tons. This large-scale stockpiling of phosphogypsum not only poses a serious threat to the ecological environment but also restricts the development of phosphate chemical enterprises. Therefore, there is an urgent need for large-scale disposal and resource utilization of this bulk solid waste, phosphogypsum.

[0003] Phosphogypsum's main component is calcium sulfate dihydrate (CaSO4·2H2O), commonly used as a cement retarder, building gypsum product, and soil conditioner. However, due to impurities such as iron, aluminum, phosphorus, and fluorine, its resource utilization faces problems such as low added value, limited application scope, and insufficient disposal capacity, making large-scale high-value utilization difficult. In contrast, anhydrous calcium sulfate (CaSO4) has advantages such as good stability, low hygroscopicity, and controllable performance. Its economic value and application potential are significantly higher than those of calcium sulfate dihydrate. Therefore, converting phosphogypsum into anhydrous calcium sulfate is an important development direction for achieving its high-value utilization. Currently, the main processes for preparing anhydrous calcium sulfate from phosphogypsum include pyrolysis and wet crystallization. Pyrolysis usually requires the removal of crystal water under high temperature conditions, which generally suffers from high energy consumption, large equipment investment, and low heat and mass transfer efficiency. In contrast, the wet process has advantages such as mild reaction conditions, strong process controllability, and low overall cost, and is currently the main development direction for preparing anhydrous calcium sulfate from phosphogypsum, with the sulfuric acid system being the most mature. However, this process inevitably leads to the expansion of the acidic aqueous solution (CaSO4·2H2O→CaSO4), resulting in a large amount of acidic leachate. This often requires chemical precipitation to remove impurity ions such as iron, aluminum, phosphorus, and fluorine to meet industrial wastewater discharge standards. However, in strongly acidic environments, problems such as high reagent consumption and difficulty in impurity removal are common. Even more challenging is that the precipitate product often reverts to being predominantly (CaSO4·2H2O), with a phase composition and physicochemical properties essentially identical to solid waste phosphogypsum. This leads to a "solid waste cycle" in the acidic leachate treatment process, hindering effective resource utilization. The reason for this phenomenon lies in the large amount of Fe in the system.2+ This lowers the nucleation free energy of calcium sulfate dihydrate and raises its dehydration energy barrier, thereby promoting the preferential formation of the crystalline phase (CaSO4·2H2O), i.e., Fe. 2+ The abundant presence of Fe favors the reaction leading to the formation of calcium sulfate dihydrate. Furthermore, Fe... 2+ Hydrolysis occurs, forming hydroxide [Fe(OH)2] precipitate. The pH is extremely high (pH>8.5), and hydroxide precipitate cannot be formed in acidic systems, resulting in a sharp increase in alkali consumption. This significantly increases the difficulty of iron removal, exacerbating the treatment cost and environmental burden of the wet process. Summary of the Invention

[0004] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing a method for the directional generation of anhydrous calcium sulfate during the purification process of phosphogypsum wet leaching solution. This method introduces an oxidant and a composite additive during the purification process of the acidic leaching solution, thereby achieving efficient removal of iron and related impurities while simultaneously regulating the crystal phase transformation behavior of calcium sulfate and promoting the directional generation of anhydrous calcium sulfate (CaSO4). This enables the entire industrial chain of CaSO4 preparation through wet crystal transformation of phosphogypsum, providing technical support for the high-value utilization of phosphogypsum.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for the directional generation of anhydrous calcium sulfate during the impurity removal process of phosphogypsum wet leaching solution, the method being as follows: S1. Under the condition of 25~100℃, while stirring the acidic leachate of the wet crystallization process of phosphogypsum, an oxidant is added to carry out an oxidation reaction to obtain an oxidized acidic leachate. S2. Add the pH adjuster at a temperature of 40~80℃ to the oxidized acidic leachate obtained in S1 in an intermittent feeding manner (addition rate of 0.1~2.0mL / min) until the pH value of the system is 3.0~4.0. Stir for 2.5~10min to obtain the pH-adjusted acidic leachate. In this step, after adding the pH adjuster and stirring continuously for a certain period of time, precipitation begins. Fe 2+ It is oxidized into a large amount of Fe 3+ In the acidic system, it rapidly precipitates out in the form of ferrous alum (potassium ferrous alum / sodium ferrous alum, XFe3[SO4(OH)3]2, X=K / Na) and ferric hydroxide [Fe(OH)3]. These large molecular aggregates carry away impurities such as phosphorus, fluorine, and aluminum in the solution through co-precipitation and adsorption, thereby achieving efficient and simultaneous removal of impurities from the leachate. S3. Add the combined additives to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 10-15 minutes, then age at 15-35℃ for 4-12 hours, and obtain the precipitate after solid-liquid separation. Dry the precipitate to obtain anhydrous calcium sulfate. The combined additives are a mixture of nucleation regulators and polyol water structure regulators.

[0006] Preferably, the impurity elements in the acidic leachate of the wet crystallization process of phosphogypsum in S1 include Fe, Al, F, and P, and the ionic form of the impurity element Fe includes Fe2+. 2+ The ionic form of Fe in this invention includes Fe2+. 2+ and Fe 3+ , with Fe 2+ The main component is sulfuric acid, which has a mass fraction of 1% to 15% in the acidic leachate of the wet crystallization process of phosphogypsum.

[0007] Preferably, the stirring speed in S1 is 300~500 rpm.

[0008] Preferably, the oxidant in S1 is one or more of H2O2, KMnO4, and Na2S2O8; the oxidant and the Fe in the acidic leachate of the wet crystallization process of the phosphogypsum are... 2+ The molar ratio is (0.5~3):1.

[0009] Preferably, the oxidation reaction time in S1 is 5-60 min.

[0010] Preferably, the pH adjuster in S2 is a calcium oxide solution or a calcium hydroxide solution; the liquid-to-solid ratio of the pH adjuster is (1-2.5):1.

[0011] Preferably, the nucleation regulator in S3 is one or more of polyacrylate, polymethyl methacrylate and polycarboxylate; the polyol water structure regulator in S3 is one or more of ethylene glycol, propylene glycol and glycerol.

[0012] Preferably, the nucleation regulator in S3 and the Ca in the acidic leachate from the wet crystallization process of phosphogypsum in S1 are... 2+ The molar ratio is (0.005~0.05):1.

[0013] Preferably, the mass ratio of the polyol-based water structure regulator in S3 to the acidic leachate from the wet crystallization process of phosphogypsum in S1 is (0.1%~5.0%):1.

[0014] Preferably, the drying conditions in S3 are: 80~300℃, 8~24h.

[0015] Compared with the prior art, the present invention has the following advantages: 1. This invention aims to solve the technical challenges of removing impurities from the acidic leachate during the wet crystallization process of phosphogypsum, including high consumption of cleaning reagents and the easy re-precipitation of chemically precipitated products as calcium sulfate dihydrate (CaSO4·2H2O). By introducing an oxidant and a composite additive during the purification process of the acidic leachate, the efficient removal of iron and related impurities is achieved while simultaneously regulating the phase transformation behavior of calcium sulfate, promoting the directional formation of anhydrous calcium sulfate (CaSO4). This enables the entire industrial chain of CaSO4 preparation from phosphogypsum through wet crystallization, providing technical support for the high-value utilization of phosphogypsum. Specifically, the acidic leachate generated during the wet crystallization process of phosphogypsum is placed in a reaction vessel and heated to a certain temperature. Then, a certain amount of oxidant is added to the system to remove the Fe, which is difficult to hydrolyze and precipitate in the solution. 2+ Oxidation treatment is performed to convert it into strongly hydrolyzable Fe. 3+ After oxidation for a certain period, an alkaline calcium-containing substance is used as a pH adjuster. It is added via a pre-treated hydrothermal slurry, introduced into the reaction system slowly or intermittently in steps, to achieve gradual and controllable pH adjustment. Subsequently, after continuous stirring for a certain period, Fe... 3+ In the initial acidic system, it rapidly precipitates in the form of ferrous alum (potassium ferrous alum / sodium ferrous alum, XFe3[SO4(OH)3]2, X=K / Na) and ferric hydroxide [Fe(OH)3]. Through co-precipitation and adsorption, it carries away impurities such as phosphorus, fluorine, and aluminum from the solution, thereby achieving efficient and simultaneous removal of impurities from the leachate.

[0016] Finally, during the aforementioned iron oxidation and precipitation process, the nucleation and growth of calcium sulfate crystals were precisely controlled by introducing a combined additive consisting of polycarboxylate nucleation regulators and polyol water structure regulators. This is because the polycarboxylate additives react with Ca through polycarboxyl groups. 2+The complexation effect and selective adsorption of specific crystal planes of CaSO4·2H2O enhance the nucleation free energy of CaSO4·2H2O and inhibit its preferential growth. Furthermore, polyol additives reduce the stability of water of crystallization by reconstructing the hydrogen bond network of water molecules in the solution, and synergistically work with polycarboxylic acid additives to weaken the nucleation energy barrier of CaSO4, promoting the preferential generation of CaSO4 crystal nuclei and guiding their directional growth. By adding a certain amount of the combined additives and stirring for a certain period, and then controlling the precipitation temperature and time, an anhydrous calcium sulfate product with stable composition and high crystallinity is finally obtained. In summary, the novel "oxidative iron removal-synergistic impurity removal-directional crystal phase control" method proposed in this invention has a simple process, significantly reduces the difficulty of removing iron and related impurities from acidic leachates and the amount of reagents consumed, and can achieve directional and synchronous generation of CaSO4 during the impurity removal process of acidic leachates, providing new technical support for the entire industrial chain of CaSO4 preparation by wet crystallization of phosphogypsum.

[0017] 2. Regarding the issue of not controlling the iron valence state (Fe) during the acidic leaching solution purification process in the traditional wet crystallization of phosphogypsum... 2+ Direct alkalization causes the precipitate to re-precipitate as calcium sulfate dihydrate (CaSO4·2H2O), a component of phosphogypsum, resulting in low impurity removal efficiency, high reagent consumption, and repeated solid-phase deposition. This invention significantly reduces the degree of alkalization required for the precipitation of iron and related impurities by introducing oxidation control methods, achieving efficient removal of iron, aluminum, phosphorus, fluorine, and other impurities with low alkali consumption. More importantly, the introduction of the oxidant significantly increases the redox potential, compresses the thermodynamic stability range of CaSO4·2H2O, and weakens the Fe... 2+ Its nucleation-promoting effect fundamentally prevents the regeneration of CaSO4·2H2O, creating favorable conditions for the directional transformation of calcium sulfate into the crystal-free phase.

[0018] 3. This invention overcomes the technical barrier of calcium sulfate easily forming a dicrystalline form and making it difficult to obtain anhydrous calcium sulfate during the impurity removal process of the acidic system in the wet crystallization of phosphogypsum. By introducing a combined additive composed of polycarboxylic acid nucleation regulators and polyol water structure regulators, the nucleation and crystallization behavior of calcium sulfate is synergistically regulated, achieving the directional generation of anhydrous calcium sulfate in the acidic system and obtaining anhydrous calcium sulfate product with a complete crystal structure and good stability. The invention not only realizes the synergistic coupling of the impurity removal process of acidic leaching solution and the crystal phase regulation of calcium sulfate, transforming the impurity removal process from a single purification step into a directional generation process of high-value product (anhydrous calcium sulfate), but also significantly improves the resource utilization level of the entire wet crystallization process of phosphogypsum.

[0019] 4. This invention achieves the controllable precipitation of anhydrous calcium sulfate by directionally regulating the phase transformation of calcium sulfate crystals. The resulting product has high purity and excellent whiteness, with the CaSO4 content typically reaching 90% to 95% and the whiteness reaching 80 to 90.

[0020] The present invention will be further described in detail below with reference to the embodiments. Detailed Implementation

[0021] Example 1 This embodiment focuses on the acidic leachate discharged during the continuous and stable leaching-washing process of a wet phosphogypsum crystallization process. The acidic leachate from this wet phosphogypsum crystallization process contains 8.5% sulfuric acid by mass, and the main impurity elements include Fe, Al, F, and P, with concentrations of 317.8 mg / L, 245.8 mg / L, 85.4 mg / L, and 69.7 mg / L, respectively. Fe is primarily present as Fe²⁺. 2+ It exists in form.

[0022] The method for the directional generation of anhydrous calcium sulfate during the impurity removal process of the wet leaching solution of phosphogypsum in this embodiment is as follows: S1. The acidic leachate from the wet crystallization process of phosphogypsum is placed in a constant-temperature reactor. At 60°C, the acidic leachate is stirred at 300 rpm while an oxidant (H2O2) is added for 10 minutes to achieve the desired Fe content in the system. 2+ The oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are fully oxidized to obtain an oxidized acidic leachate; the Fe in the oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are... 2+ The molar ratio is 0.5:1; S2 adds a pH adjuster (calcium oxide solution, liquid-solid ratio of 2:1) at a rate of 1.5 mL / min to the acidic leachate obtained in S1 after oxidation, in an intermittent feeding manner (to be completed within 60 min) until the pH value of the system is 3.5. Stir for 5 min to obtain the acidic leachate after pH adjustment. In this step, after adding the pH adjuster and stirring continuously for a certain period of time, precipitation begins. Fe 2+ It is oxidized into a large amount of Fe 3+ In the acidic system, it rapidly precipitates out in the form of ferrous alum (potassium ferrous alum / sodium ferrous alum, XFe3[SO4(OH)3]2, X=K / Na) and ferric hydroxide [Fe(OH)3]. These large molecular aggregates carry away impurities such as phosphorus, fluorine, and aluminum in the solution through co-precipitation and adsorption, thereby achieving efficient and simultaneous removal of impurities from the leachate. S3. Add the combined additives to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 10 min, then let it stand and age for 8 h at a temperature of 25℃, and then perform solid-liquid separation to obtain the precipitate. After drying the precipitate at 120℃ for 12 h, anhydrous calcium sulfate is obtained. The combined additive is a mixture of a nucleation regulator (sodium polyacrylate) and a polyol-based water structure regulator (ethylene glycol); The nucleation regulator reacts with the Ca in the acidic leachate from the wet crystallization process of phosphogypsum in S1. 2+ The molar ratio is 0.01:1; The mass ratio of the polyol-based water structure regulator to the acidic leachate from the wet crystallization process of phosphogypsum in S1 is 1.0%:1.

[0023] In this embodiment, the removal rates of Fe, Al, F and P in the acidic leachate of the wet crystallization process of phosphogypsum were 92.4%, 93.4%, 87.4% and 85.6%, respectively, and the final anhydrous calcium sulfate had a purity of 92% and a whiteness of 86.

[0024] The anhydrous calcium sulfate prepared in this embodiment is a product of controlled precipitation of anhydrous calcium sulfate by directional regulation of the calcium sulfate crystal phase transformation. The resulting product has high purity and excellent whiteness, which is significantly better than the conventional wet crystallization product of phosphogypsum.

[0025] Example 2 This embodiment focuses on the acidic leachate discharged during the continuous and stable leaching-washing process of a wet phosphogypsum crystallization process. The acidic leachate from this wet phosphogypsum crystallization process contains 15% sulfuric acid by mass, and the main impurity elements include Fe, Al, F, and P, with concentrations of 350.5 mg / L, 278.4 mg / L, 78.4 mg / L, and 71.5 mg / L, respectively. Fe is primarily present as Fe²⁺. 2+ It exists in form.

[0026] The method for the directional generation of anhydrous calcium sulfate during the impurity removal process of the wet leaching solution of phosphogypsum in this embodiment is as follows: S1. The acidic leachate from the wet crystallization process of phosphogypsum is placed in a constant-temperature reactor. At 25°C, the acidic leachate is stirred at 500 rpm while an oxidant (KMnO4) is added for 60 minutes to achieve the desired Fe content in the system. 2+ The oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are fully oxidized to obtain an oxidized acidic leachate; the Fe in the oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are... 2+ The molar ratio is 3:1; S2. Add the pH adjuster (calcium oxide solution, liquid-solid ratio of 1:1) at a temperature of 40℃ to the acidic leachate obtained in S1 at an addition rate of 0.1 mL / min in an intermittent feeding manner (to be completed within 60 min) until the pH value of the system is 3.0. Stir for 2.5 min to obtain the acidic leachate after pH adjustment. S3. Add the combined additives to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 15 min, then let it stand and age for 12 h at 15℃, and then perform solid-liquid separation to obtain the precipitate. After drying the precipitate at 150℃ for 12 h, anhydrous calcium sulfate is obtained. The combined additive is a mixture of a nucleation regulator (sodium polymethacrylate) and a polyol-based water structure regulator (propylene glycol); The nucleation regulator reacts with the Ca in the acidic leachate from the wet crystallization process of phosphogypsum in S1. 2+ The molar ratio is 0.005:1; The mass ratio of the polyol-based water structure regulator to the acidic leachate from the wet crystallization process of phosphogypsum in S1 is 0.1%:1.

[0027] In this embodiment, the removal rates of Fe, Al, F and P in the acidic leachate of the wet crystallization process of phosphogypsum were 93.5%, 92.4%, 87.4% and 87.6%, respectively, and the final anhydrous calcium sulfate had a purity of 93% and a whiteness of 85.

[0028] Example 3 This embodiment focuses on the acidic leachate discharged during the continuous and stable leaching-washing process of a wet phosphogypsum crystallization process. The acidic leachate from this wet phosphogypsum crystallization process contains 1% sulfuric acid by mass, and the main impurity elements include Fe, Al, F, and P, with concentrations of 424.1 mg / L, 294.7 mg / L, 86.9 mg / L, and 80.4 mg / L, respectively. Fe is primarily present as Fe²⁺. 2+ It exists in form.

[0029] The method for the directional generation of anhydrous calcium sulfate during the impurity removal process of the wet leaching solution of phosphogypsum in this embodiment is as follows: S1. Place the acidic leachate from the wet crystallization process of phosphogypsum in a constant-temperature reactor. At 100℃, stir the acidic leachate at 400 rpm while adding an oxidant (Na2S2O8) for 5 minutes to achieve Fe... 2+ The oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are fully oxidized to obtain an oxidized acidic leachate; the Fe in the oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are... 2+ The molar ratio is 2:1; S2. Add the pH adjuster (calcium hydroxide solution, liquid-solid ratio of 2.5:1) at a temperature of 70℃ to the acidic leachate obtained in S1 at an addition rate of 2.0 mL / min in an intermittent feeding manner (to be completed within 60 min) until the pH value of the system is 4.0. Stir for 10 min to obtain the acidic leachate after pH adjustment. S3. Add the combined additives to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 12 min, then let it stand and age for 4 h at a temperature of 35℃, and then perform solid-liquid separation to obtain the precipitate. After drying the precipitate at 80℃ for 24 h, anhydrous calcium sulfate is obtained. The combined additive is a mixture of a nucleation regulator (sodium polycarboxylate) and a polyol-based water structure regulator (glycerol); The nucleation regulator reacts with the Ca in the acidic leachate from the wet crystallization process of phosphogypsum in S1. 2+ The molar ratio is 0.05:1; The mass ratio of the polyol-based water structure regulator to the acidic leachate from the wet crystallization process of phosphogypsum in S1 is 5.0%:1.

[0030] In this embodiment, the removal rates of Fe, Al, F and P in the acidic leachate of the wet crystallization process of phosphogypsum were 93.8%, 92.4%, 88.4% and 82.9%, respectively, and the final anhydrous calcium sulfate had a purity of 93% and a whiteness of 83.

[0031] Example 4 This embodiment focuses on the acidic leachate discharged during the continuous and stable leaching-washing process of a wet phosphogypsum crystallization process. The acidic leachate from this wet phosphogypsum crystallization process contains 5.0% sulfuric acid by mass, and the main impurity elements include Fe, Al, F, and P, with concentrations of 384.9 mg / L, 284.3 mg / L, 84.9 mg / L, and 82.1 mg / L, respectively. Fe is primarily present as Fe²⁺. 2+ It exists in form.

[0032] The method for the directional generation of anhydrous calcium sulfate during the impurity removal process of the wet leaching solution of phosphogypsum in this embodiment is as follows: S1. The acidic leachate from the wet crystallization process of phosphogypsum is placed in a constant-temperature reactor. At 80°C, the acidic leachate is stirred at 350 rpm while an oxidant (a mixture of H2O2 and Na2S2O8 in a 1:1 mass ratio) is added to induce an oxidation reaction for 50 minutes, thereby achieving the desired Fe content in the system. 2+ The oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are fully oxidized to obtain an oxidized acidic leachate; the Fe in the oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are...2+ The molar ratio is 2.5:1; S2. Add the pH adjuster (calcium oxide solution, liquid-solid ratio of 1.5:1) at a rate of 1.5 mL / min in an intermittent feeding manner (to be completed within 60 min) to the acidic leachate obtained in S1 after oxidation until the pH value of the system is 3.0. Stir for 8 min to obtain the acidic leachate after pH adjustment. S3. Add the combined additives to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 12 min, let it stand and age for 8 h at 20℃, and then perform solid-liquid separation to obtain the precipitate. Dry the precipitate at 250℃ for 10 h to obtain anhydrous calcium sulfate. The combined additive is a mixture of nucleation regulator (a mixture of ammonium polyacrylate and ammonium polymethacrylate in a mass ratio of 1:1) and polyol water structure regulator (a mixture of ethylene glycol and propylene glycol in a mass ratio of 2:1). The nucleation regulator reacts with the Ca in the acidic leachate from the wet crystallization process of phosphogypsum in S1. 2+ The molar ratio is 0.03:1; The mass ratio of the polyol-based water structure regulator to the acidic leachate from the wet crystallization process of phosphogypsum in S1 is 4%:1.

[0033] In this embodiment, the removal rates of Fe, Al, F and P in the acidic leachate of the wet crystallization process of phosphogypsum were 93.2%, 91.8%, 86.9% and 85.1%, respectively, and the final anhydrous calcium sulfate had a purity of 93% and a whiteness of 84.

[0034] Example 5 This embodiment focuses on the acidic leachate discharged during the continuous and stable leaching-washing process of a wet phosphogypsum crystallization process. The acidic leachate from this wet phosphogypsum crystallization process contains 7.5% sulfuric acid by mass, and the main impurity elements include Fe, Al, F, and P, with concentrations of 389.4 mg / L, 298.5 mg / L, 84.9 mg / L, and 82.7 mg / L, respectively. Fe is primarily present as Fe²⁺. 2+ It exists in form.

[0035] The method for the directional generation of anhydrous calcium sulfate during the impurity removal process of the wet leaching solution of phosphogypsum in this embodiment is as follows: S1. The acidic leachate from the wet crystallization process of phosphogypsum is placed in a constant-temperature reactor. At 30°C, the acidic leachate is stirred at 450 rpm while an oxidant (a mixture of KMnO4 and Na2S2O8 with a molar ratio of 0.75:1) is added to induce an oxidation reaction for 40 minutes, thereby achieving the desired Fe content in the system.2+ The oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are fully oxidized to obtain an oxidized acidic leachate; the Fe in the oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are... 2+ The molar ratio is 1:1; In this embodiment, the oxidant may also be one or more of H2O2, KMnO4 and Na2S2O8; S2. At an addition rate of 1.8 mL / min, a pH adjuster (calcium hydroxide solution, liquid-solid ratio of 1.5:1) at a temperature of 50℃ is added to the acidic leachate obtained in S1 after oxidation until the pH value of the system is 3.5. Stir for 6 min to obtain the acidic leachate after pH adjustment. S3. Add the combined additives to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 10 min, then let it stand and age for 5 h at 30℃, and then perform solid-liquid separation to obtain the precipitate. After drying the precipitate at 300℃ for 8 h, anhydrous calcium sulfate is obtained. The combined additive is a mixture of nucleation regulator (a mixture of sodium polymethyl methacrylate and sodium polycarboxylate in a mass ratio of 1:2) and polyol water structure regulator (a mixture of propylene glycol and glycerol in a mass ratio of 2:1). In this embodiment, the nucleation regulator can also be one of polyacrylate, polymethacrylate, and polycarboxylate (the cation in the salt is Na). + Or NH4 + ); Polyol water structure modifiers can also be one or more of ethylene glycol, propylene glycol, and glycerol; The nucleation regulator reacts with the Ca in the acidic leachate from the wet crystallization process of phosphogypsum in S1. 2+ The molar ratio is 0.03:1; The mass ratio of the polyol-based water structure regulator to the acidic leachate from the wet crystallization process of phosphogypsum in S1 is 3%:1.

[0036] In this embodiment, the removal rates of Fe, Al, F and P in the acidic leachate of the wet crystallization process of phosphogypsum were 94.1%, 90.9%, 83.4% and 84.1%, respectively, and the final anhydrous calcium sulfate had a purity of 92% and a whiteness of 83.

[0037] Comparative Example 1 The method for the directional generation of anhydrous calcium sulfate in the wet leaching process of phosphogypsum in this comparative example is the same as in Example 1. The source of the acidic leaching solution in the wet crystallization process of phosphogypsum is the same as in Example 1. The difference is that no oxidant (hydrogen peroxide) was added to the acidic leaching solution to remove Fe during the impurity removal process. 2+ Oxidation treatment is performed, and other steps are the same as in Example 1. The specific method is as follows: S1. The acidic leachate from the wet crystallization process of phosphogypsum is placed in a constant temperature reactor. Under the condition of 60℃, a pH adjuster (calcium oxide solution, liquid-solid ratio of 2:1) at 80℃ is added to the oxidized acidic leachate obtained in S1 at an addition rate of 1.5mL / min in an intermittent feeding manner (to be completed within 60min) until the pH value of the system is 3.5. Stir for 5min to obtain the acidic leachate after pH adjustment. S3. Add the combined additives to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 10 min, and then let it stand and age for 8 h at a temperature of 25℃. The combined additive is a mixture of a nucleation regulator (sodium polyacrylate) and a polyol-based water structure regulator (ethylene glycol); The nucleation regulator reacts with the Ca in the acidic leachate from the wet crystallization process of phosphogypsum in S1. 2+ The molar ratio is 0.01:1; The mass ratio of the polyol-based water structure regulator to the acidic leachate from the wet crystallization process of phosphogypsum in S1 is 1.0%:1.

[0038] The results showed that, without Fe 2+ Under conditions of oxidation regulation, Fe 2+ Hydrolysis is difficult to occur at the aforementioned endpoint pH (precipitation begins at pH > 8.5). Therefore, under these conditions, the removal rates of Fe, Al, F, and P in the solution are only about 19.4%, 26.8%, 16.8%, and 21.8%, respectively, significantly lower than in Example 1; to achieve a comparable removal effect, the alkali consumption would increase dramatically. More importantly, the chemical precipitate generated in this process is calcium sulfate dihydrate (CaSO4·2H2O), which cannot achieve the directional formation of anhydrous calcium sulfate (CaSO4). In summary, without the control of iron valence state oxidation, the impurity removal process is not only inefficient and consumes a large amount of reagents, but also easily leads to the repeated precipitation of phosphogypsum components (CaSO4·2H2O), and cannot achieve the directional formation of anhydrous calcium sulfate during the impurity removal process.

[0039] Comparative Example 2 The method for the directional generation of anhydrous calcium sulfate in the wet leaching process of phosphogypsum in this comparative example is the same as in Example 1. The source of the acidic leaching solution in the wet crystallization process of phosphogypsum is the same as in Example 1. The difference is that after completing the oxidation and iron removal and pH adjustment, the combination additive of sodium polyacrylate and ethylene glycol is no longer added to synergistically regulate the directional generation of anhydrous calcium sulfate crystals. Other steps are the same as in Example 1. The specific method is as follows: S1. Place the acidic leachate from the wet crystallization process of phosphogypsum in a constant-temperature reactor. While stirring the acidic leachate at 300 rpm, add the oxidant (H2O2) for 10 minutes to achieve the desired Fe content in the system. 2+ The oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are fully oxidized to obtain an oxidized acidic leachate; the Fe in the oxidizing agent and the acidic leachate from the wet crystallization process of phosphogypsum are... 2+ The molar ratio is 0.5:1; S2. Add the pH adjuster (calcium oxide solution, liquid-solid ratio of 2:1) at a rate of 1.5 mL / min in an intermittent feeding manner (to be completed within 60 min) to the acidic leachate obtained in S1 after oxidation until the pH value of the system is 3.5. Stir for 5 min to obtain the acidic leachate after pH adjustment. S3. After stirring the acidic leachate obtained in S2 with adjusted pH for 10 minutes, let it stand and age for 8 hours at 25°C.

[0040] The results showed that, without the introduction of combined additives to regulate the nucleation and crystal growth process of calcium sulfate, the removal rates of Fe, Al, F, and P in the solution were 90.1%, 91.3%, 82.6%, and 80.4%, respectively. This indicates that oxidation for iron removal is beneficial for reducing alkali consumption and improving impurity purification efficiency. However, the calcium sulfate precipitate generated during the same precipitation process was mainly composed of hemihydrate calcium sulfate (CaSO4·0.5H2O) and dihydrate calcium sulfate (CaSO4·2H2O), and failed to be directionally generated as anhydrous calcium sulfate (CaSO4).

[0041] In summary, even when impurities such as iron, aluminum, fluorine, and phosphorus are removed in an acidic system without the synergistic regulation of combined additives, it is difficult to suppress the formation of hydrated calcium sulfate and obtain anhydrous calcium sulfate products with stable composition and high crystallinity. This indicates that combined additives play an irreplaceable key role in achieving the directional formation of anhydrous calcium sulfate during the impurity removal process.

[0042] In summary, this invention achieves the controllable precipitation of anhydrous calcium sulfate by directionally regulating the phase transformation of calcium sulfate crystals. The resulting product has high purity and excellent whiteness, with CaSO4 content typically reaching 90%–95% and whiteness reaching 80–90, which is significantly superior to conventional wet crystallization products of phosphogypsum.

[0043] 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 the directional generation of anhydrous calcium sulfate during the impurity removal process of phosphogypsum wet leaching solution, characterized in that, The method is as follows: S1. Under conditions of 25~100℃, while stirring the acidic leachate from the wet crystallization process of phosphogypsum, an oxidizing agent is added to carry out an oxidation reaction, resulting in an oxidized acidic leachate; the impurity elements in the acidic leachate from the wet crystallization process of phosphogypsum in S1 include Fe, Al, F and P, and the ionic form of the impurity element Fe includes Fe2+. 2+ The sulfuric acid mass fraction in the acidic leachate of the wet crystallization process of phosphogypsum is 1%~15%; the oxidant in S1 is one or more of H2O2, KMnO4 and Na2S2O8; the Fe in the oxidant and the acidic leachate of the wet crystallization process of phosphogypsum... 2+ The molar ratio is (0.5~3):1; S2. Add a pH adjuster at a temperature of 40-80℃ to the oxidized acidic leachate obtained in S1 at an addition rate of 0.1-2.0 mL / min in an intermittent feeding manner until the pH value of the system is 3.0-4.

0. Stir for 2.5-10 min to obtain the pH-adjusted acidic leachate. The pH adjuster in S2 is a calcium oxide solution or a calcium hydroxide solution. The liquid-to-solid ratio of the pH adjuster is (1-2.5):

1. S3. Add the combined additive to the acidic leachate obtained in S2 after adjusting the pH value, stir and react for 10-15 min, then age at 15-35℃ for 4-12 h, and perform solid-liquid separation to obtain a precipitate. Dry the precipitate to obtain anhydrous calcium sulfate. The combined additive is a mixture of a nucleating agent and a polyol-based water structure regulator. The nucleating agent in S3 is one or more of polyacrylate, polymethacrylate, and polycarboxylate. The polyol-based water structure regulator is one or more of ethylene glycol, propylene glycol, and glycerol. The nucleating agent in S3 reacts with the calcium in the acidic leachate from the wet crystallization process of phosphogypsum in S1. 2+ The molar ratio is (0.005~0.05):1; the mass ratio of the polyol water structure regulator in S3 to the acidic leachate of the wet crystallization process of phosphogypsum in S1 is (0.1%~5.0%):

1.

2. The method for directional generation of anhydrous calcium sulfate during the wet leaching process of phosphogypsum according to claim 1, characterized in that, The stirring speed in S1 is 300~500 rpm.

3. The method for directional generation of anhydrous calcium sulfate during the wet leaching process of phosphogypsum according to claim 1, characterized in that, The oxidation reaction in S1 takes 5 to 60 minutes.

4. The method for directional generation of anhydrous calcium sulfate during the wet leaching process of phosphogypsum according to claim 1, characterized in that, The drying conditions in S3 are: 80~300℃, 8~24h.