Harmless conversion treatment method of phosphogypsum

Abstract

The invention belongs to the technical field of industrial solid waste treatment, and particularly relates to a harmless conversion treatment method of ardealite, which comprises the following steps of: preparing potassium ammonium gypsum solid solution double salt from ardealite, K2SO4 and (NH4) 2SO4 through a one-step synthesis process, and mutually replacing K < + > and NH4 < + > in any proportion to form a continuous solid solution in a crystallization process by utilizing the characteristics that the K < + > and the NH4 < + > are similar in ion radius and coordination; and constructing a stable double salt structure together with Ca < 2 + >, SO4 < 2-> and crystal water. The solid solution double salt can slowly release K < + > and NH4 < + > in soil, the potassium-ammonium ratio of the product is regulated and controlled by adjusting the ratio of K2SO4 to (NH4) 2SO4, the differentiated requirements of different crops on nutrients are met, a solid-liquid quantitative model is established based on a quaternary system phase diagram, a mother liquor circulation production process is performed, it is ensured that the system is stably located in a solid solution double salt crystalline region, the purity and consistency of the product are guaranteed, and the quality of the product is improved. And high-valued conversion and large-scale consumption of the phosphogypsum are realized.

Description

Technical Field

[0001] This invention relates to the field of industrial solid waste treatment technology, and in particular to a method for the harmless transformation and treatment of phosphogypsum. Background Technology

[0002] Phosphogypsum, a major byproduct of the phosphate fertilizer industry, has a massive annual production scale, with my country alone discharging tens of millions of tons annually. For a long time, stockpiling has been the primary method of disposal, not only consuming vast amounts of land resources but also causing significant leakage due to residual acidity, fluorides, and trace heavy metals carried by the phosphogypsum itself, under the influence of rainwater leaching and natural weathering. This leads to soil acidification and compaction, groundwater deterioration, and seriously threatens regional ecological security and human health, becoming a core pain point hindering the green and sustainable transformation of the phosphate chemical industry. Although traditional phosphogypsum utilization methods cover multiple fields such as building material production, roadbed backfilling, and soil improvement, these methods generally suffer from low added value and limited utilization rates. They fail to fundamentally resolve the long-term crisis of phosphogypsum stockpiling and fail to achieve high-value-added and circular utilization of phosphogypsum resources. Furthermore, with the deepening of the concept of precision fertilization in modern agriculture, different crops have shown significant differences in their requirements for nitrogen and potassium nutrients. However, most existing agricultural nitrogen-potassium compound fertilizers are physically mixed products, which have the drawback of not matching crop needs. This not only easily leads to nutrient loss and low fertilizer utilization, increasing agricultural production costs, but may also cause secondary environmental problems such as eutrophication of water bodies due to nutrient excess. Although some studies have attempted to transform phosphogypsum into an agricultural nutrient carrier and explore new pathways for the harmless transformation of phosphogypsum, most processes cannot simultaneously achieve efficient phosphogypsum utilization and flexible control of the nutrient ratio of the product, which seriously limits its large-scale application in the agricultural field. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for the harmless transformation and treatment of phosphogypsum. This method involves preparing a potassium ammonium gypsum solid solution complex salt from phosphogypsum, K₂SO₄, and (NH₄)₂SO₄ through a one-step synthesis process, utilizing K₂SO₄... + With NH4 + The similarity in ionic radii and coordination allows them to substitute for each other in any proportion during crystallization to form a continuous solid solution, and to interact with Ca. 2+ SO4 2- Together with water of crystallization, they form a stable complex salt structure. Solid solution complex salts can slowly release potassium in soil. + With NH4 +Furthermore, by adjusting the ratio of K2SO4 to (NH4)2SO4, the potassium-ammonium ratio of the product can be controlled to adapt to the different nutrient requirements of different crops. Based on the quaternary system phase diagram, a solid-liquid quantitative model is established to carry out the mother liquor recycling production process, ensuring that the system is stably in the solid solution double salt crystallization zone, ensuring product purity and consistency, and realizing the high-value conversion and large-scale consumption of phosphogypsum.

[0004] To achieve the above objectives, the present invention provides a method for the harmless transformation and treatment of phosphogypsum, comprising the following steps:

[0005] S1. Allow phosphogypsum to age naturally in a well-ventilated and dry environment, add CaO powder, stir until the pH stabilizes at 6.5-7.5, dry and sieve to obtain pretreated phosphogypsum.

[0006] S2. Mix pretreated phosphogypsum, K2SO4 and (NH4)2SO4, add deionized water according to the liquid-solid ratio, stir the reaction to obtain a reaction mixture containing potassium ammonium gypsum solid solution double salt.

[0007] S3. The reaction mixture containing potassium ammonium gypsum solid solution double salt is subjected to hydraulic separation to obtain an upper liquid and a lower liquid. The mixture is filtered and vacuum filtered. The upper liquid yields potassium ammonium gypsum solid solution double salt crystals, while the lower liquid yields insoluble impurities. The liquid produced during the filtration process is collected as ammonium-rich mother liquor.

[0008] In one feasible implementation, in step S1, the aging time is 36-60 hours; the mass of the CaO powder is 0.6% of the mass of phosphogypsum; the stirring rate is 140-160 r / min; the drying temperature is 60-80°C; the drying time is 2-4 hours; and the sieve mesh size is 80 mesh.

[0009] Phosphogypsum raw materials often contain residual acidic substances and unstable physical structures. The pretreatment process, through natural aging, improves the particle uniformity and structural stability of the phosphogypsum, creating a favorable physical foundation for subsequent reactions. CaO, as an alkaline regulator, can neutralize the residual acids in the phosphogypsum. By adjusting the system pH to a neutral range, the adverse effects of an acidic environment on subsequent double salt crystallization are eliminated, preventing damage to product purity. A drying step removes free water from the system, reducing its interference with the mixing uniformity and reaction efficiency of the raw materials. Further sieving removes large, difficult-to-react impurities or clumps from the phosphogypsum, ultimately yielding pretreated phosphogypsum with stable physicochemical properties and low impurity content, providing a high-quality raw material guarantee for efficient subsequent synthesis reactions and improved product quality.

[0010] In one feasible implementation, in step S2, the mass ratio of the pretreated phosphogypsum, K2SO4, and (NH4)2SO4 is 12:(45-80):(48-95); the liquid-solid ratio is (4-5):1; the temperature of the stirring reaction is 323.2K; the stirring reaction rate is 250-300 r / min; and the stirring reaction time is 80-100 min.

[0011] The synthesis reaction uses pretreated phosphogypsum, K₂SO₄, and (NH₄)₂SO₄ as core reactants. A multi-component ionic reaction environment is constructed in an aqueous solution system. Utilizing the free movement of ions in aqueous solution, K₂SO₄ dissociates into K₂SO₄... + NH4+ dissociated from (NH4)2SO4 + With the Ca provided by pretreated phosphogypsum 2+ Together with SO4 2- A synergistic effect occurs, forming a potassium ammonium gypsum solid solution complex salt through a co-crystallization process. This is achieved by utilizing the Ca in phosphogypsum... 2+ It is transformed into a high-value solid solution complex salt product, realizing the resource utilization and high-value use of phosphogypsum. At the same time, through the directional binding between ions, it provides a reaction mixture rich in the target product for subsequent separation and purification steps, laying the material basis for product recovery.

[0012] In one feasible implementation, in step S3, the hydraulic separation step is as follows: the reaction mixture containing potassium ammonium gypsum solid solution double salt is pumped into the feed inlet of a hydrocyclone at a flow rate of 0.8-1.2 m / s, the overflow velocity of the hydrocyclone is controlled at 0.3-0.5 m / s, the underflow velocity is controlled at 0.1-0.2 m / s, and the separation time is 5-10 min; the filtration medium is qualitative filter paper, the filtration speed is 5-10 mL / min; the vacuum filtration pressure is 0.06-0.08 MPa, and the vacuum filtration time is 15-20 min.

[0013] Hydraulic separation utilizes the differences in particle size and density between the potassium ammonium gypsum solid solution double salt crystals and insoluble impurities, causing them to naturally separate into layers within the system, achieving initial solid-liquid separation and impurity enrichment. Subsequent filtration and vacuum filtration, through media retention and pressure difference, further remove residual fine impurities from the system, improving the purity of the target product. Simultaneously, the ammonium-rich mother liquor collected during vacuum filtration is valuable because it contains unreacted potassium... + NH4 + SO4 2- Collecting effective ions and mother liquor provides a reserve of reusable resources for subsequent recycling processes, preventing the loss of effective components with waste liquid. This reduces raw material waste and provides core raw materials for circular production, achieving a closed-loop resource system for the process.

[0014] In one feasible implementation, the method further includes a mother liquor recycling process: determining the K content in the ammonium-rich mother liquor obtained from S3. + NH4 + SO4 2- The concentration of the target product is determined based on the potassium-ammonium ratio. The mass of the added raw material is calculated according to the solid-liquid quantitative model formula established by the quaternary system phase diagram. The added raw material is added to the ammonium-rich mother liquor. The resulting mixture is used as the reaction solvent for S2. S2 and S3 are repeated for cyclic synthesis and separation.

[0015] By measuring the concentration of effective ions in the ammonium-rich mother liquor and combining it with the potassium-ammonium ratio requirements of the target product, a quantitative relationship between the liquid-phase ion ratio and the solid-phase product ratio is established using a solid-liquid quantitative model. This allows for the calculation of the type and amount of supplementary raw materials, ensuring that the ion composition of the system remains within the threshold range for the formation of double salts in the solid solution, thus avoiding the formation of single double salts that could affect product quality. This process not only reduces the consumption of fresh raw materials and improves resource utilization and process economy by recycling effective ions from the mother liquor, but also enables flexible switching of the product's potassium-ammonium ratio through precise model control. This breaks the limitation of a single product ratio in traditional circular production, adapting to the needs of different application scenarios while ensuring the stability and consistency of the product in each cycle, ultimately achieving continuous, efficient, and environmentally friendly large-scale production.

[0016] In one feasible implementation, the quaternary system phase diagram is K. + NH4 + Ca 2+ / / SO4 2- -H2O, the temperature of the quaternary system phase diagram is 323.2K; the expression of the solid-liquid quantitative model is: y=7.28·(x-0.0378) / (2.342-x), where x is the K in the liquid phase. + With NH4 + The molar ratio, y is the K content in the solid product. + With NH4 + The molar ratio; the solid-liquid quantitative model is applicable to the liquid phase composition conditions for the formation of the potassium ammonium gypsum solid solution complex salt, wherein K in the liquid phase + With NH4 + The molar ratio x is greater than 0.0378 and less than 2.342.

[0017] In one feasible implementation, the general chemical formula of the potassium ammonium gypsum is [K x (NH4) 1-x ]2SO4·CaSO4·H2O, where the value of x ranges from 0.11 to 0.84.

[0018] K + With NH4 +With similar ionic radii, the same charge number, and similar coordination environments, the two form a solid solution in the crystal lattice. Under certain process conditions, the stable range of the solid solution is 0.11-0.84. By optimizing the reaction temperature, the uniformity of raw material mixing, and the precise feeding strategy in the mother liquor circulation, the stable phase region boundary of the solid solution complex salt in the quaternary system phase diagram can be further broadened. When x approaches 0.01, ammonium ions are absolutely dominant, or when x approaches 0.99, potassium ions are absolutely dominant. A small number of heterogeneous ions can still stably occupy the cation sites in the crystal lattice, forming a solid solution complex salt with a uniform structure.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] This scheme uses phosphogypsum, with CaSO4·2H2O as its core raw material, to prepare potassium ammonium gypsum solid solution complex salt through a one-step synthesis process, achieving high-value transformation and large-scale utilization of phosphogypsum. + and NH4 + Because of their similar ionic radii and coordination characteristics, they can substitute for each other in any proportion and occupy the same type of cation sites during crystallization, forming a continuous solid solution phase. Meanwhile, Ca... 2+ With SO4 2- Together with water of crystallization, they participate in lattice construction, combining with the solid solution to form a structurally stable potassium ammonium gypsum solid solution complex salt, enabling the large-scale utilization of phosphogypsum. Furthermore, the solid solution complex salt can slowly dissociate in moist soil environments, continuously releasing potassium, essential for crop growth. + With NH4 + Furthermore, by adjusting the ratio of K₂SO₄ and (NH₄)₂SO₄ in the raw materials, the potassium-ammonium ratio in the solid solution double salt can be controlled, thereby adapting to the differentiated nitrogen and potassium nutrient requirements of different crops. This achieves the harmless and high-value transformation of phosphogypsum from industrial solid waste into a high-value agricultural nutrient carrier. Based on this, a K₂SO₄-based... + NH4 + Ca 2+ / / SO4 2- The solid-liquid quantitative model of the H2O quaternary system phase diagram is used to carry out a mother liquor recycling production process. The ion-rich mother liquor after a single synthesis is reused as the reaction medium. The raw materials are accurately added through model calculation, so that the system is always in the stable crystallization zone of solid solution double salt. This avoids the formation of single double salt impurities, ensures product purity and consistency, improves raw material utilization, and provides a solution with both ecological and economic value for the green transformation of the phosphorus chemical industry and targeted fertilization in agriculture. Attached Figure Description

[0021] Figure 1This is a scanning electron microscope image of the solid solution complex salt potassium ammonium gypsum prepared in Example 1 of the present invention.

[0022] Figure 2 For the embodiment of the present invention, K is 323.2K. + NH4 + Ca 2+ / / SO4 2- -H2O quaternary system composition phase diagram.

[0023] Figure 3 The image shows the XRD pattern of the solid solution complex salt potassium ammonium gypsum prepared in Example 1 of this invention.

[0024] Figure 4 The image shows the XRD pattern of the solid solution complex salt potassium ammonium gypsum prepared in Example 3 of this invention.

[0025] Figure 5 The infrared image is of the solid solution complex salt potassium ammonium gypsum prepared in Example 1 of this invention. Detailed Implementation

[0026] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the application will be further described in detail below with reference to embodiments. However, this should not be construed as limiting the scope of this application to the following examples. All other embodiments obtained by those skilled in the art without creative effort without departing from the above-described methodological spirit of this application are within the scope of protection of this application.

[0027] The singular forms “for,” “or,” “a,” “any,” and “described” used in this application are intended to include the plural forms unless the context clearly indicates otherwise. Furthermore, the terms “first” and “second” are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0028] Example 1

[0029] A method for harmlessly transforming phosphogypsum includes the following steps:

[0030] S1. Naturally age the phosphogypsum raw material in a ventilated and dry environment for 48 hours. Add CaO powder at a ratio of 0.6 wt% of the phosphogypsum mass and mechanically stir at a rate of 150 r / min. During this period, monitor the pH value of the system with a pH meter every 5 minutes and continue stirring until the pH value stabilizes at 7.0. After neutralization, dry at a temperature of 70℃ for 3 hours and pass through an 80-mesh standard sieve to obtain pretreated phosphogypsum.

[0031] S2. Mix 12g of pretreated phosphogypsum, 80g of K2SO4 and 48g of (NH4)2SO4, add deionized water at a liquid-to-solid ratio of 4.5:1, and continue the reaction for 90 minutes at a temperature of 323.2K and a stirring rate of 280r / min to obtain a reaction mixture containing potassium ammonium gypsum solid solution double salt.

[0032] S3. The reaction mixture containing potassium ammonium gypsum solid solution double salt was pumped into the feed inlet of a hydrocyclone at a flow rate of 1.0 m / s. The overflow velocity of the hydrocyclone was controlled at 0.4 m / s and the underflow velocity at 0.15 m / s. Hydraulic separation was carried out for 7 min. The upper layer was a clear liquid containing potassium ammonium gypsum solid solution double salt crystals, and the lower layer was a turbid liquid containing insoluble impurities. The upper and lower liquids were initially filtered with qualitative filter paper at a flow rate of 7 mL / min. Then, the two filtrates were vacuum filtered for 18 min under a filtration pressure of 0.07 MPa. The potassium ammonium gypsum solid solution double salt crystals were collected from the filtration product of the upper filtrate, and the insoluble impurities were separated from the filtration product of the lower filtrate. At the same time, the ammonium-rich mother liquor generated during the vacuum filtration process was collected, and the temperature of the mother liquor was maintained at 323.2 K.

[0033] The solid phase composition of the prepared potassium ammonium gypsum solid solution double salt crystals is [K 0.84 (NH4) 0.16 The reaction 2SO4·CaSO4·H2O produces approximately 3.2 mol of phosphogypsum per mole of the above potassium ammonium gypsum product.

[0034] Scanning electron microscope image of the prepared solid solution complex salt potassium ammonium gypsum is shown below. Figure 1 As shown, it is needle-shaped and has an excellent aspect ratio.

[0035] Infrared image of the prepared solid solution complex salt potassium ammonium gypsum is shown below. Figure 5 As shown.

[0036] Example 2

[0037] A method for harmlessly transforming phosphogypsum includes the following steps:

[0038] S1. Naturally age the phosphogypsum raw material in a ventilated and dry environment for 36 hours. Add CaO powder at a ratio of 0.6 wt% of the phosphogypsum mass and mechanically stir at a rate of 140 r / min. During this period, monitor the pH value of the system with a pH meter every 5 minutes and continue stirring until the pH value stabilizes at 6.5. After neutralization, dry at a temperature of 60℃ for 2 hours and pass through an 80-mesh standard sieve to obtain pretreated phosphogypsum.

[0039] S2. Mix 12g of pretreated phosphogypsum, 50g of K2SO4 and 87g of (NH4)2SO4, add deionized water at a liquid-to-solid ratio of 4:1, and continue the reaction for 80 minutes at a temperature of 323.2K and a stirring rate of 250r / min to obtain a reaction mixture containing potassium ammonium gypsum solid solution double salt.

[0040] S3. The reaction mixture containing potassium ammonium gypsum solid solution double salt was pumped into the feed inlet of a hydrocyclone at a flow rate of 0.8 m / s. The overflow velocity of the hydrocyclone was controlled at 0.3 m / s and the underflow velocity at 0.1 m / s. Hydraulic separation was carried out for 5 min. The upper layer was a clear liquid containing potassium ammonium gypsum solid solution double salt crystals, and the lower layer was a turbid liquid containing insoluble impurities. The upper and lower layers were initially filtered with qualitative filter paper at a flow rate of 5 mL / min. Then, the two filtrates were vacuum filtered for 15 min at a filtration pressure of 0.06 MPa. The potassium ammonium gypsum solid solution double salt crystals were collected from the filtration product of the upper filtrate, and the insoluble impurities were separated from the filtration product of the lower filtrate. At the same time, the ammonium-rich mother liquor generated during the vacuum filtration process was collected, and the temperature of the mother liquor was maintained at 323.2 K.

[0041] The solid phase composition of the prepared potassium ammonium gypsum solid solution double salt crystals is [K 0.51 (NH4) 0.49 The reaction 2SO4·CaSO4·H2O produces approximately 3.1 mol of phosphogypsum per mole of the above potassium ammonium gypsum product.

[0042] Example 3

[0043] A method for harmlessly transforming phosphogypsum includes the following steps:

[0044] S1. Naturally age the phosphogypsum raw material in a ventilated and dry environment for 60 hours. Add CaO powder at a ratio of 0.6 wt% of the phosphogypsum mass and mechanically stir at a rate of 160 r / min. During this period, monitor the pH value of the system with a pH meter every 5 minutes and continue stirring until the pH value stabilizes at 7.5. After neutralization, dry at a temperature of 80℃ for 4 hours and pass through an 80-mesh standard sieve to obtain pretreated phosphogypsum.

[0045] S2. Mix 12g of pretreated phosphogypsum, 45g of K2SO4 and 95g of (NH4)2SO4, add deionized water at a liquid-to-solid ratio of 5:1, and continue the reaction for 100min at a temperature of 323.2K and a stirring rate of 300r / min to obtain a reaction mixture containing potassium ammonium gypsum solid solution double salt.

[0046] S3. The reaction mixture containing potassium ammonium gypsum solid solution double salt was pumped into the feed inlet of a hydrocyclone at a flow rate of 1.2 m / s. The overflow velocity of the hydrocyclone was controlled at 0.5 m / s and the underflow velocity at 0.2 m / s. Hydraulic separation was carried out for 10 min. The upper layer was a clear liquid containing potassium ammonium gypsum solid solution double salt crystals, and the lower layer was a turbid liquid containing insoluble impurities. The upper and lower layers were initially filtered with qualitative filter paper at a flow rate of 10 mL / min. Then, the two filtrates were vacuum filtered for 20 min at a filtration pressure of 0.08 MPa. The potassium ammonium gypsum solid solution double salt crystals were collected from the filtration product of the upper filtrate, and the insoluble impurities were separated from the filtration product of the lower filtrate. At the same time, the ammonium-rich mother liquor generated during the vacuum filtration process was collected, and the temperature of the mother liquor was maintained at 323.2 K.

[0047] The solid phase composition of the prepared potassium ammonium gypsum solid solution double salt crystals is [K 0.27 (NH4) 0.73 The reaction 2SO4·CaSO4·H2O produces approximately 3.3 mol of phosphogypsum per mole of the above potassium ammonium gypsum product.

[0048] Comparative Example 1

[0049] A method for harmless transformation and treatment of phosphogypsum, the implementation steps and parameters of which differ from those of Example 1 are that the amounts of pretreated phosphogypsum, K2SO4 and (NH4)2SO4 are changed to 10g, 75g and 17g respectively.

[0050] The prepared product is a single complex salt rather than a solid solution complex salt, and the solid phase composition is K2SO4·CaSO4·H2O.

[0051] Comparative Example 2

[0052] A method for harmless transformation and treatment of phosphogypsum, the implementation steps and parameters of which differ from those of Example 1 are that the amounts of pretreated phosphogypsum, K2SO4 and (NH4)2SO4 are changed to 12g, 12g and 140g respectively.

[0053] The prepared product is a single complex salt rather than a solid solution complex salt, and the solid phase composition is (NH4)2SO4·CaSO4·H2O.

[0054] The raw material quantities and solid and liquid phase composition data of the product preparation process in Examples 1-3 and Comparative Examples 1-2 are summarized in Table 1.

[0055] Table 1. Composition of the products prepared in Examples 1-3 and Comparative Examples 1-2

[0056]

[0057] As shown in Table 1, different compositions of solid solution complex salts can be obtained by adding raw materials with different potassium-ammonium ratios.

[0058] like Figure 3 As shown in the XRD pattern of the solid solution complex salt potassium ammonium gypsum prepared in Example 1, the product [K 0.84 (NH4) 0.16 The XRD pattern of K₂SO₄·CaSO₄·H₂O closely matches the peak positions of the standard cards for K₂SO₄·CaSO₄·H₂O and (NH₄)₂SO₄·CaSO₄·H₂O, with no obvious impurity peaks, indicating that the product is a single phase. Compared to the standard card for K₂SO₄·CaSO₄·H₂O, the characteristic peaks are slightly shifted to a lower angle, which is due to a small amount of larger NH₄⁺ ions. + Replaced K in the crystal lattice + This leads to an increase in the unit cell parameter, further proving that the product is K. + With NH4 + A continuous substitutional solid solution is formed.

[0059] like Figure 4 As shown in the XRD pattern of the solid solution complex salt potassium ammonium gypsum prepared in Example 3, the product [K 0.27 (NH4) 0.73 The measured spectrum of the product K₂SO₄·CaSO₄·H₂O closely matches the peak positions of the standard cards for K₂SO₄·CaSO₄·H₂O and (NH₄)₂SO₄·CaSO₄·H₂O, with no obvious impurity peaks, indicating that the product is a single phase. Compared to the standard card for (NH₄)₂SO₄·CaSO₄·H₂O, its characteristic peaks show a slight shift towards higher angles. This is due to a small amount of K₂SO₄·CaSO₄·H₂O with smaller radii in the product. + It replaced NH4 in the crystal lattice + This leads to a decrease in the unit cell parameter, which also proves that the product is K. + With NH4 + A continuous substitutional solid solution is formed.

[0060] Table 2 K under 323.2K + NH4 + Ca 2+ / / SO4 2- Table of Liquid Phase Dry Salt Composition and Solid Product Type Correspondence for Phase Diagram Data Points in the H2O Quaternary System

[0061]

[0062] Table 2 shows the corresponding... Figure 2 323.2K in the middle and K in the lower range + NH4 + Ca 2+ / / SO4 2-Key data points of the H2O quaternary system phase diagram; the core of the phase diagram presents three types of stable phase regions: the target potassium ammonium gypsum solid solution complex salt [K x (NH4) 1-x The table shows the stable crystallization region of phosphogypsum (K2SO4·CaSO4·H2O), the single complex salt K2SO4·CaSO4·H2O phase region, and the (NH4)2SO4·CaSO4·H2O phase region. The boundaries of the solid solution phase regions correspond to threshold ranges of J(K2SO4) 6%-80% and J[(NH4)2SO4] 20%-93%. The data in Table 2 further validates these threshold ranges. Data points within the solid solution phase region have liquid phase compositions that meet the threshold requirements, visually demonstrating the quantitative correlation between liquid phase composition and solid phase products. This provides a core theoretical basis for establishing a solid-liquid quantitative model, ensuring the process stability and product reliability of the entire process of high-value conversion and large-scale consumption of phosphogypsum.

[0063] Based on the five sets of experimental data from Examples 1-3 and Comparative Examples 1-2 in Table 1, the ratio of K₂SO₄ to (NH₄)₂SO₄ in the liquid phase and the K₂SO₄ content in the resulting solid solution double salt were established. + With NH4 + The quantitative relationship between the proportions. Among these, due to the extremely low solubility of CaSO4·2H2O, the main component of phosphogypsum, the free Ca in the salt solution of the reaction system... 2+ Since the concentration is negligible, the derivation of the quantitative relationship focuses only on the interaction between dissolved K2SO4 and (NH4)2SO4.

[0064] The mass standard of dry salt in the liquid phase is set as 100g. Dry salt refers to salt dissolved in the solution. The mass of K2SO4 is m1 (relative molecular mass M1), and the mass of (NH4)2SO4 is m2 (relative molecular mass M2), and m1+m2≈100 is satisfied.

[0065] K in liquid phase + With NH4 + molar ratio:

[0066] Further, through the conversion of mass ratio,

[0067] K in solids + With NH4 + molar ratio:

[0068] like Figure 2 As shown, K at 323.2K + NH4 + Ca 2+ / / SO4 2-The phase diagram of the H2O quaternary system indicates that, according to the univariate curves O1O3 and O2O6, the products corresponding to these single-variable curves are single complex salts rather than solid-solution complex salts, respectively, corresponding to K in the solid products. + NH4 + =2:0 and K + NH4 + The ratio is 0:2, therefore the above quantitative relationship has a clear threshold constraint:

[0069] 1) When the mass fraction of K2SO4 in the liquid phase J(K2SO4) > 80% and the mass fraction of (NH4)2SO4 J[(NH4)2SO4] < 20%, the liquid phase molar ratio x > 2.342. In this case, the product is a single double salt K2SO4·CaSO4·H2O, and the K2SO4 content in the solid phase is... + As the proportion approaches 1, y→+∞;

[0070] 2) When the mass fraction of K2SO4 in the liquid phase J(K2SO4) < 6% and the mass fraction of (NH4)2SO4 J[(NH4)2SO4] > 93%, the liquid phase molar ratio x < 0.0378. At this point, the product is a single double salt (NH4)2SO4·CaSO4·H2O, and the K2SO4 content in the solid phase is... + When the percentage approaches 0, y=0.

[0071] The equation is obtained as follows:

[0072] set up ,

[0073] Substituting the data into Table 1 yields the proportionality coefficient.

[0074] Therefore, we obtain the formula: ,

[0075] Therefore, the expression for the solid-liquid quantitative model is: y = 7.28·(x-0.0378) / (2.342-x), where x is the K in the liquid phase. + With NH4 + The molar ratio, y is the K content in the solid product. + With NH4 + The molar ratio.

[0076] Based on the solid-liquid quantitative model obtained above, the ammonium-rich mother liquor obtained from a single synthesis will be further processed into a recycling process:

[0077] The collected ammonium-rich mother liquor was analyzed for composition, and the potassium content in the mother liquor was determined. + NH4 + SO4 2-The concentration, combined with the potassium-ammonium ratio of the target product, is substituted into the K-based formula. + NH4 + Ca 2+ / / SO4 2- The solid-liquid quantitative model established by the phase diagram of the H2O quaternary system was used to calculate the specific mass of pretreated phosphogypsum, K2SO4 or (NH4)2SO4 to be added. The added raw materials were added to the ammonium-rich mother liquor and stirred until the system was clear. Then the mixture was repeated with the process parameters of the one-step synthesis reaction to continue the co-crystallization reaction. After each cycle of reaction, the separation and purification steps were repeated and the product was collected.

[0078] To verify the applicability of this model in continuous cyclic production, the mother liquor obtained in Example 2 was reintroduced into the reaction system as the initial reaction medium. The calculated raw material ratios are shown in Table 3. According to the raw material ratios corresponding to groups 6 and 7 in Table 3, the corresponding masses of pretreated phosphogypsum, K₂SO₄, and (NH₄)₂SO₄ were added to the mother liquor. After reacting for 90 minutes at a constant temperature of 323.2 K and a stirring rate of 280 r / min, the composition of both the mother liquor and the solid phase showed a regular change: the K₂SO₄ content in the mother liquor increased... + With NH4 + After adjustments to the materials in the 6th and 7th experiments, the molar ratio of the liquid phase approached the target liquid phase molar ratio x in both experiments. Meanwhile, due to the continuous addition of pretreated phosphogypsum, the system remained within the stable region of the solid solution complex salt in the quaternary phase diagram, without deviating to the single complex salt phase region.

[0079] Table 3. Raw material ratios and solid phase composition of the mother liquor recycling production process

[0080]

[0081] Substitute the actual measured values ​​from the experiments in Tables 1 and 3 into the established quantitative model formula for calculation, and compare the calculation results with the theoretical prediction values. The results are shown in Table 4. The data show that the relative deviation of each experimental point is less than 0.05, indicating that the model has high reliability and prediction accuracy.

[0082] Table 4. Relative deviations between experimental values ​​and theoretical calculations

[0083]

[0084] As shown in Table 4, the cyclic data of the experiments in groups 6 and 7 were substituted into the solid-liquid quantitative correlation model for verification. The results showed that the relative deviation of the two groups of experiments was less than 5.0%. This result fully confirms that the model has good practical applicability and precise control capability in the mother liquor circulation process. It also provides direct experimental basis for flexibly switching the potassium-ammonium ratio of the product according to the nutrient requirements of different crops and realizing large-scale cyclic production.

[0085] Through the mother liquor recycling process, the effective ions (K) in the reaction system + NH4 + SO4 2- The recovery rate of phosphogypsum was significantly improved, and raw material loss was reduced. At the same time, the amount of phosphogypsum consumed for each mole of potassium ammonium gypsum solid solution double salt product generated remained stable between 3.1 and 3.3 moles, continuing the core advantage of this technology for large-scale phosphogypsum utilization.

[0086] In summary, this recycling process relies on the same mother liquor system and, through precise material accounting and model guidance, achieves targeted and flexible control of the potassium-ammonium ratio in the product. It breaks through the limitation of traditional recycling processes that can only produce products with a fixed composition, and provides reliable technical support for adapting to the differentiated nutrient requirements of different crops and for industrial-scale production.

[0087] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0088] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A method for the harmless transformation and treatment of phosphogypsum, characterized in that, Includes the following steps: S1. The phosphogypsum is naturally aged in a well-ventilated and dry environment, CaO powder is added, stirred, dried and sieved to obtain pretreated phosphogypsum. S2. Mix pretreated phosphogypsum, K2SO4 and (NH4)2SO4, add deionized water according to the liquid-solid ratio, stir the reaction to obtain a reaction mixture containing potassium ammonium gypsum solid solution double salt. S3. The reaction mixture containing potassium ammonium gypsum solid solution double salt is subjected to hydraulic separation to obtain an upper liquid and a lower liquid. The mixture is filtered and vacuum filtered. The upper liquid yields potassium ammonium gypsum solid solution double salt crystals, while the lower liquid yields insoluble impurities. The liquid produced during the filtration process is collected as ammonium-rich mother liquor.

2. The method for harmless transformation and treatment of phosphogypsum according to claim 1, characterized in that, In step S1, the aging time is 36-60 hours; the mass of the CaO powder is 0.6% of the mass of phosphogypsum; the stirring rate is 140-160 r / min; the drying temperature is 60-80°C; the drying time is 2-4 hours; and the sieve mesh size is 80 mesh.

3. The method for harmless transformation and treatment of phosphogypsum according to claim 1, characterized in that, In step S2, the mass ratio of the pretreated phosphogypsum, K2SO4, and (NH4)2SO4 is 12:(45-80):(48-95); the liquid-solid ratio is (4-5):1; the temperature of the stirring reaction is 323.2K; the stirring reaction rate is 250-300 r / min; and the stirring reaction time is 80-100 min.

4. The method for harmless transformation and treatment of phosphogypsum according to claim 1, characterized in that, In step S3, the hydraulic separation step is as follows: the reaction mixture containing potassium ammonium gypsum solid solution double salt is pumped into the feed inlet of the hydrocyclone at a flow rate of 0.8-1.2 m / s, the overflow port water flow velocity of the hydrocyclone is controlled at 0.3-0.5 m / s, the underflow port water flow velocity is controlled at 0.1-0.2 m / s, and the separation time is 5-10 min.

5. The method for harmless transformation and treatment of phosphogypsum according to claim 1, characterized in that, In step S3, the filtration medium is qualitative filter paper, the filtration rate is 5-10 mL / min, the vacuum filtration pressure is 0.06-0.08 MPa, and the vacuum filtration time is 15-20 min.

6. The method for harmless transformation and treatment of phosphogypsum according to claim 1, characterized in that, The method further includes a mother liquor recycling process: determining the K content in the ammonium-rich mother liquor obtained from S3. + NH4 + SO4 2- The concentration of the target product is determined based on the potassium-ammonium ratio. The mass of the added raw material is calculated according to the solid-liquid quantitative model formula established by the quaternary system phase diagram. The added raw material is added to the ammonium-rich mother liquor. The resulting mixture is used as the reaction solvent for S2. S2 and S3 are repeated for cyclic synthesis and separation.

7. The method for harmless transformation and treatment of phosphogypsum according to claim 6, characterized in that, The phase diagram of the quaternary system is K. + NH4 + Ca 2+ / / SO4 2- -H2O, the temperature of the quaternary system phase diagram is 323.2K.

8. The method for harmless transformation and treatment of phosphogypsum according to claim 6, characterized in that, The expression for the solid-liquid quantitative model is: y = 7.28·(x-0.0378) / (2.342-x), where x is the K in the liquid phase. + With NH4 + The molar ratio, y is the K content in the solid product. + With NH4 + The molar ratio.

9. The method for harmless transformation and treatment of phosphogypsum according to claim 8, characterized in that, The solid-liquid quantitative model is applicable to the liquid phase composition conditions for the formation of the potassium ammonium gypsum solid solution complex salt, wherein K in the liquid phase + With NH4 + The molar ratio x is greater than 0.0378 and less than 2.

342.

10. The method for harmless transformation and treatment of phosphogypsum according to claim 1, characterized in that, The general chemical formula of the potassium ammonium gypsum is [K x (NH4) 1-x ]2SO4·CaSO4·H2O, where the value of x ranges from 0.11 to 0.84.

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