A method for directional recovery of fluorapatite from phosphogypsum leachate

Abstract

The application belongs to the technical field of phosphogypsum resource utilization, and particularly relates to a method for directionally recovering fluorapatite from phosphogypsum leaching solution. The method comprises the following steps: performing rough filtration and flocculation pretreatment on the phosphogypsum leaching solution to remove suspended solid matters; then adding an alkaline precipitant to adjust the pH to 6-12 to remove precipitates; then adding a Ca source according to a Ca:P:F molar ratio of (4.8-5.2):(2.8-3.2):(0.8-1.2), or simultaneously adding the Ca source and a P or F source, adjusting the pH to a favorable crystallization interval of 7-9, introducing fluorapatite crystal seeds to stir and crystallize or to stir and crystallize by cooling, and performing solid-liquid separation, so that the obtained solid phase is Ca5(PO4)3F. Through specific synergistic regulation, conditioning, adjustment and induced crystallization process, the method can directionally recover high-purity fluorapatite from the phosphogypsum leaching solution.

Description

Technical Field

[0001] This invention belongs to the field of phosphogypsum resource utilization technology, specifically relating to a method for the targeted recovery of fluorapatite from phosphogypsum leaching solution. Background Technology

[0002] Phosphogypsum (mainly composed of CaSO4·2H2O) is a major industrial byproduct of wet-process phosphoric acid production. Approximately 4.5 to 5.5 tons of phosphogypsum are generated for every ton of H3PO4 (phosphoric acid) produced. Globally, the cumulative stockpile of phosphogypsum has exceeded 6 billion tons, making the development of corresponding resource utilization and treatment technologies increasingly urgent. The remaining components of phosphogypsum mainly contain phosphorus (P2O5 content 0.84%~1.22%), fluorine (F content 0.13%~0.33%), and other impurities such as SiO2, Al2O3, and Fe2O3. The leaching solution contains both recoverable valuable components and impurities that affect downstream applications (metal ions, organic matter, silicates, etc.). Recovering phosphorus and fluorine resources can not only alleviate environmental pressure but also be converted into fertilizers, building materials, or chemical raw materials, supporting the circular economy and the United Nations Sustainable Development Goals (SDGs).

[0003] Currently disclosed technologies for treating phosphogypsum leachate primarily utilize precipitation, crystallization, ion exchange, or adsorption to recover fluorine or phosphorus. Common recovered products include calcium fluoride (CaF2), fluorosilicates, and phosphate precipitates. For instance, Chinese invention patent CN116651906 A discloses the use of amorphous, hydroxyl-containing, and isoelectric point less than 6.5 iron-manganese-aluminum ternary metal oxides as adsorbents to treat acid-treated phosphogypsum leachate when its pH is ≤6 and SO42- content is low. 2- Phosphorus and fluorine adsorption treatment was carried out under conditions with a content of 0.02–0.12 mol / L to achieve the resource utilization of phosphogypsum. Chinese invention patent with publication number CN117800379 A discloses the use of a curing agent to solidify the phosphogypsum leaching solution, followed by the addition of polyaluminum ferric sulfate for flocculation treatment. This allows soluble phosphorus and fluorine in the phosphogypsum leaching solution to enter the hydrolytic precipitate generated by polyaluminum ferric sulfate, promoting the flocculation and removal of soluble phosphorus and fluorine in the leaching solution.

[0004] Fluorapatite (Ca5(PO4)3F, FA), a major component of phosphate rock, is of high value. If recovered from phosphogypsum leaching solution, it can be reused in phosphoric acid production, reducing raw material consumption. However, existing technologies do not disclose the overall process and parameters for selectively obtaining high-purity fluorapatite directly from the leaching solution and reusing the purified solution in the phosphogypsum washing process, nor do they provide a complete and clearly defined industrially applicable implementation method. Summary of the Invention

[0005] In view of the shortcomings and deficiencies of the existing technology, the purpose of this invention is to provide a method for the targeted recovery of fluorapatite from phosphogypsum leaching solution.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for the targeted recovery of fluorapatite from phosphogypsum leaching solution includes the following steps:

[0008] (1) Pretreatment of raw liquid: The phosphogypsum leaching liquid is subjected to coarse filtration and flocculation pretreatment to remove suspended solids;

[0009] (2) Synergistic regulation: Add an alkaline precipitant to the pretreatment solution in step (1) to adjust the pH to 6-12, stir to carry out the precipitation reaction, and filter to remove the precipitate;

[0010] (3) Preparation and conditioning: Determination of Ca in the solution after step (2) 2+ PO4 3- With F - Activity, add Ca source according to Ca:P:F molar ratio of (4.8~5.2):(2.8~3.2):(0.8~1.2) (close to 5:3:1), or add Ca source and P or F source simultaneously (if needed), and then adjust pH to the crystallization favorable range of 7~9;

[0011] (4) Induced crystallization: Fluoroapatite seed crystals are introduced into the solution after step (3) and stirred to crystallize or cooled and stirred to crystallize. Solid-liquid separation is performed, and the obtained solid phase is Ca5(PO4)3F(FA).

[0012] Furthermore, the coarse filtration mentioned in step (1) refers to coarse filtration using a sieve or sand filter with a pore size of 50~200μm. Coarse filtration pretreatment can effectively separate larger particulate solids and reduce the amount of flocculant used in subsequent flocculation pretreatment.

[0013] Further, the flocculation pretreatment mentioned in step (1) refers to adding polyacrylamide flocculant at a concentration of 5~15 mg / L for flocculation pretreatment, and then removing suspended solids by cyclone classification or sedimentation after flocculation pretreatment. Flocculation pretreatment can effectively separate small-diameter particulate solids, thereby reducing the impurity content in the induced crystallization solid phase Ca5(PO4)3F product.

[0014] Further, the alkaline precipitant mentioned in step (2) is selected from one or more of CaO, Ca(OH)2, CaCO3, K2CO3, KOH, NaOH, Na2CO3, and NaHCO3.

[0015] Furthermore, the precipitation reaction in step (2) is carried out at a temperature of 20~90℃ for 5~60 min.

[0016] Furthermore, the filtration to remove precipitates in step (2) uses a filter screen with a pore size of 1~10μm.

[0017] By employing a synergistic regulation step with an alkaline precipitant, associated impurities such as Fe, Al, Si, and Mg can be effectively precipitated, reducing the concentration of soluble impurities to the threshold required for subsequent Ca5(PO4)3F crystallization. This improves the yield and purity of the solid-phase Ca5(PO4)3F obtained from subsequent induced crystallization.

[0018] Furthermore, in step (3), the Ca source is selected from one or more of CaCl2, Ca(OH)2, and CaO; the P source is selected from phosphoric acid; and the F source is selected from one or two of NaF and KF.

[0019] Furthermore, the concentration of fluorapatite seed crystals added in step (4) is 0.2~1.0 wt%.

[0020] Furthermore, the stirring crystallization in step (4) can be carried out using equipment such as a stirred crystallizer, a continuous fluidized bed, or a circulating heat-insulating crystallizer. The stirring crystallization temperature is 20~90℃ and the time is 0.5~24 h.

[0021] By controlling the Ca:P:F molar ratio and adjusting the pH to the favorable crystallization range of 7-9 during the formulation and conditioning process, and obtaining a suitable supersaturation, it is possible to ensure that the main phase generated during crystallization is Ca5(PO4)3F(FA), while inhibiting the formation of CaF2 and Ca2+. 10 The formation of unfavorable phases such as (PO4)6(OH)2 (hydroxyapatite) and Na2SiF6 can be eliminated, thereby improving the yield and purity of the target product Ca5(PO4)3F.

[0022] Further, the solid phase from the solid-liquid separation in step (4) is washed and dried to obtain Ca5(PO4)3F; the liquid phase is filtered sequentially by microfiltration, nanofiltration, and reverse osmosis, and the resulting effluent is directly recycled in the phosphogypsum washing section. The nanofiltration concentrate is rich in sulfates, which can be explored for the recovery of chemical products such as calcium sulfate, or recycled in a targeted and quantitative manner in specific production processes. The reverse osmosis concentrate contains high concentrations of chloride ions and potassium and sodium salts, and evaporation and crystallization to produce sodium and potassium salts for recovery are considered.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] (1) The method of the present invention recovers elements such as Mg, Al, Fe, and Si from the complex matrix of the eluent (containing Ca, PO4, F, SO4, Si, Fe, Al, Mg, Na, K, Cl, etc.) through a synergistic control step, thereby reducing the concentration of soluble impurities to meet the threshold for subsequent Ca5(PO4)3F crystallization, thereby improving the yield and purity of the solid phase Ca5(PO4)3F obtained by subsequent induced crystallization.

[0025] (2) Selective induction of fluorapatite crystallization is controlled through blending and conditioning processes, rather than the formation of unfavorable phases (such as CaF2, Ca). 10 (PO4)6(OH)2, Na2SiF6, etc.), thereby improving the yield and purity of the target product Ca5(PO4)3F.

[0026] (3) While achieving the target product with high purity, ensure that the mother liquor / washing liquid can be reused for phosphogypsum washing after purification and that the circulation is stable. Attached Figure Description

[0027] Figure 1 This is a general process flow diagram of a method for the directional recovery of fluorapatite from phosphogypsum leaching solution in Example 1.

[0028] Figure 2 and Figure 3 The image shows the SEM-EDS spectrum of the precipitate obtained in step (2) of Example 1.

[0029] Figure 4 The graph shows the changes in the Mg / Fe / Al / Si element content in the eluent after precipitation reaction under different pH conditions in Example 1.

[0030] Figure 5 The image shows the XRD pattern of the secondary FA product obtained in Example 1.

[0031] Figure 6 and Figure 7 The image shows the SEM-EDS spectrum of the secondary FA product obtained in Example 1. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0033] Example 1

[0034] A method for the targeted recovery of fluorapatite from phosphogypsum leaching solution, the overall process flow diagram of which is shown below. Figure 1 As shown. Specifically, it includes the following steps:

[0035] (1) Pretreatment of raw liquid: The phosphogypsum leaching liquid (main components: Ca about 3000 mg / L, P about 4000 mg / L, F about 2500 mg / L) from a certain phosphate fertilizer plant was first coarsely filtered through 100 μm, and then 10 mg / L of polyacrylamide flocculant was added for flocculation pretreatment. After standing for 30 min, suspended solids were removed.

[0036] (2) Synergistic regulation: Na2CO3 was added to the pretreatment solution in step (1) to adjust the pH, and the mixture was stirred at 40℃ for 20 min to carry out the precipitation reaction, so as to precipitate Fe / Al / Si / Mg byproducts. The precipitate was removed by 5μm filtration. The SEM-EDS pattern of the obtained precipitate is shown below. Figure 2 and Figure 3 As shown in Table 1 below, the XRF characterization results are as follows.

[0037] Table 1. XRF characterization results of primary precipitation

[0038] element Na Mg Al Si S Cl K Fe Primary precipitation (%) 22.6 17.1 26.6 20.1 1.74 0.532 5.60 5.51

[0039] The changes in the Mg / Fe / Al / Si elemental content in the eluent after precipitation reaction under different pH conditions in this step are as follows: Figure 4 As shown. By Figure 4 The results show that Mg / Fe / Al / Si were basically completely precipitated in the eluent after the pH reached 6.

[0040] (3) Preparation and conditioning: Determination of Ca in the solution after step (2) 2+ PO4 3- With F - To determine the activity, slowly add Ca(OH)2 suspension and phosphoric acid at a Ca:P:F molar ratio of 5:3:1, raise the system temperature to 60℃, and slowly adjust the pH to 7.4 (to promote FA nucleation and growth).

[0041] (4) Induced crystallization: 0.5 wt% of pre-prepared FA seed crystals were introduced into the solution after treatment in step (3), stirred and kept at 60℃ for 6 h. The solid phase was separated by plate and frame filtration. The solution was first washed twice with an equal volume of deionized water, and then washed with a small amount of ethanol / water mixed solution to remove strongly adsorbed salts, to obtain wet FA. The wet FA was dried at 105℃ to constant weight to obtain secondary FA product, which was weighed and sampled for characterization. The XRD pattern of the obtained secondary FA product is shown below. Figure 5 As shown, the SEM-EDS map is as follows: Figure 6 and Figure 7 As shown in Table 2, the XRF characterization results are shown in Table 3.

[0042] Table 2. XRF characterization results of secondary FA products

[0043] element F Na Mg Al Si P S Cl K Ca Fe Secondary FA(%) 8.295 2.084 0.265 0.298 0.205 29.344 0.467 0.445 0.199 58.251 0.129

[0044] Table 3. ICP-OES characterization results of secondary FA products

[0045] element P <![CDATA[P2O5]]> Ca Mg Al Si Fe Secondary FA(%) 17.80 40.79 38.82 0.22 0.31 0.13 0.16

[0046] XRF and ICP-OES characterization showed that Ca in the system 2+ PO4 3- With F - The phosphorus was effectively converted and enriched in the fluorapatite crystalline phase. After treatment, the concentrations of phosphorus (P) in the filtrate decreased to <50 mg / L, phosphorus (F) to <100 mg / L, and calcium (Ca) to <200 mg / L, corresponding to P and F recoveries of approximately 98% and 96%, respectively, and a Ca recovery rate of approximately 93%. The secondary FA product is high-purity fluorapatite with a Ca / P molar ratio of 1.69, which translates to a P₂O₅ content of 40.79%, meeting the requirements of industrial-grade FA. It exhibits extremely low impurity content, a purity of approximately 96%, and excellent product quality, possessing the potential for resource utilization or industrialization.

[0047] (5) Mother liquor treatment: The filtrate from step (4) is treated by microfiltration, nanofiltration, and reverse osmosis membrane, and the effluent meets the reuse standards and is reused for phosphogypsum washing. The nanofiltration concentrate is rich in sulfates, and it is possible to explore the recovery of chemical products such as calcium sulfate, or to reuse it in specific production processes at designated locations and in specific quantities. The reverse osmosis concentrate contains high concentrations of chloride ions and potassium and sodium salts, and it is considered to evaporate and crystallize them to produce sodium and potassium salts for recovery.

[0048] Example 2

[0049] (1) Pretreatment of raw liquid: The phosphogypsum leaching liquid (main components: Ca about 3000 mg / L, P about 4000 mg / L, F about 2500 mg / L) from a certain phosphate fertilizer plant was first coarsely filtered through 50 μm, and then 5 mg / L of polyacrylamide flocculant was added for flocculation pretreatment. After standing and settling for 30 min, suspended solids were removed.

[0050] (2) Synergistic regulation: Ca(OH)2 is added to the pretreatment solution in step (1) to adjust the pH to 9.0, and the solution is stirred at 60℃ for 10 min to carry out the precipitation reaction to precipitate Fe / Al / Si / Mg concomitant organisms. The precipitate is removed by 5μm filtration.

[0051] (3) Preparation and conditioning: Determination of Ca in the solution after step (2) 2+ PO4 3- With F - To determine the activity, slowly add Ca(OH)2 suspension and phosphoric acid at a Ca:P:F molar ratio of 5:3:1, raise the system temperature to 70℃, and slowly adjust the pH to 8.0 (to promote FA nucleation and growth).

[0052] (4) Induced crystallization: 0.4 wt% of pre-prepared FA seed crystals were introduced into the solution after treatment in step (3), stirred and kept at 70°C for 4 h. The solid phase was separated by plate and frame filtration. The solid phase was first washed twice with an equal volume of deionized water, and then washed with a small amount of ethanol / water mixed solution to remove strongly adsorbed salts, to obtain wet FA. The wet FA was dried at 105°C to constant weight to obtain secondary FA product.

[0053] (5) Mother liquor treatment: The filtrate from step (4) is treated by microfiltration, nanofiltration, reverse osmosis membrane, etc. The effluent meets the reuse standards and is reused for phosphogypsum washing.

[0054] The purity of the secondary FA product obtained in this embodiment is 93%, and the recovery rates of Ca, P, and F are 94%, 97%, and 93%, respectively.

[0055] Example 3

[0056] (1) Pretreatment of raw liquid: The phosphogypsum leaching liquid (main components: Ca about 3000 mg / L, P about 4000 mg / L, F about 2500 mg / L) from a certain phosphate fertilizer plant was first coarsely filtered through 150 μm, and then 15 mg / L of polyacrylamide flocculant was added for flocculation pretreatment. The suspended solids were removed by cyclone classification.

[0057] (2) Synergistic regulation: NaOH was added to the pretreatment solution in step (1) to adjust the pH to 10.0, and the solution was stirred at 90℃ for 5 min to carry out the precipitation reaction to precipitate Fe / Al / Si / Mg concomitant organisms. The precipitate was removed by 5μm filtration.

[0058] (3) Preparation and conditioning: Determination of Ca in the solution after step (2) 2+ PO4 3- With F - To determine the activity, slowly add CaCl2 solution and phosphoric acid at a Ca:P:F molar ratio of 5:3:1, raise the system temperature to 80℃, and slowly adjust the pH to 9.0 (to promote FA nucleation and growth).

[0059] (4) Induced crystallization: 0.3 wt% of pre-prepared FA seed crystals were introduced into the solution after treatment in step (3), stirred and kept at 80°C for 2 h. The solid phase was separated by plate and frame filter press. The solid phase was first washed twice with an equal volume of deionized water, and then washed with a small amount of ethanol / water mixed solution to remove strongly adsorbed salts, to obtain wet FA. The wet FA was dried at 105°C to constant weight to obtain secondary FA product.

[0060] (5) Mother liquor treatment: The filtrate from step (4) is treated by microfiltration, nanofiltration, reverse osmosis membrane, etc. The effluent meets the reuse standards and is reused for phosphogypsum washing.

[0061] The purity of the secondary FA product obtained in this embodiment is 95%, and the recovery rates of Ca, P, and F are 95%, 91%, and 91%, respectively.

[0062] Example 4

[0063] A method for the directional recovery of fluorapatite from phosphogypsum leaching solution, compared with Example 1, wherein the pH of the preparation and conditioning step (3) is adjusted to 6.4, and the rest are the same.

[0064] The Ca / P molar ratio of the obtained secondary FA product was 1.61, the purity of the product was 92%, and the recovery rates of Ca, P and F were 88%, 92% and 91%, respectively.

[0065] Example 5

[0066] A method for the directional recovery of fluorapatite from phosphogypsum leaching solution, compared with Example 1, wherein the pH of the preparation and conditioning step (3) is adjusted to 10.0, and the rest are the same.

[0067] The Ca / P molar ratio of the obtained secondary FA product was 1.75, the purity of the product was 90%, and the recovery rates of Ca, P and F were 92%, 95% and 95%, respectively.

[0068] Comparative Examples 1-3

[0069] Comparative Examples 1-3 are identical to Examples 1-3 except for step (2) the synergistic regulation step.

[0070] Comparative Examples 1-3, without pretreatment to remove Mg, Al, Fe, and Si impurities from the eluent, underwent direct fluorapatite-induced crystallization, resulting in a significant decrease in system recovery efficiency and product quality. Specifically, the characterization results showed that P and F in the treated wastewater remained at 250-400 mg / L and 150-250 mg / L, respectively, corresponding to a removal rate of only about 80%, significantly lower than the >96% level achievable with pretreatment. The resulting solid product contained only about 20%-25% P₂O₅ and 6%-8% F, significantly lower than typical fluorapatite indicators, and the Ca / P molar ratio decreased to 1.45-1.55, indicating significant lattice substitution and structural defects. XRD analysis showed a significant reduction in product crystallinity. Simultaneously, the formation of large amounts of Al / Fe(OH)₃ and Si colloids led to particle refinement, increasing the filter cake moisture content to 45%-60%, significantly increasing filtration resistance and severely impacting solid-liquid separation performance. The results show that if impurity ions such as Mg, Al, Fe, and Si are not removed beforehand, direct induced crystallization not only makes it difficult to obtain high-quality fluorapatite products, but also significantly reduces the recovery efficiency of P and F and the operability of the process.

[0071] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for the targeted recovery of fluorapatite from phosphogypsum leaching solution, characterized in that... Includes the following steps: (1) Pretreatment of raw liquid: The phosphogypsum leaching liquid is subjected to coarse filtration and flocculation pretreatment to remove suspended solids; (2) Synergistic regulation: Add an alkaline precipitant to the pretreatment solution in step (1) to adjust the pH to 6-12, stir to carry out the precipitation reaction, and filter to remove the precipitate; (3) Preparation and conditioning: Determination of Ca in the solution after step (2) 2+ PO4 3- With F - To determine the activity, add a Ca source at a Ca:P:F molar ratio of (4.8~5.2):(2.8~3.2):(0.8~1.2), or simultaneously add a Ca source and a P or F source, and then adjust the pH to the favorable crystallization range of 7~9. (4) Induced crystallization: Fluoroapatite seed crystals are introduced into the solution after step (3) and stirred to crystallize or cooled and stirred to crystallize. Solid-liquid separation is performed, and the obtained solid phase is Ca5(PO4)3F.

2. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The filtration mentioned in step (1) refers to coarse filtration using a sieve or sand filter with a pore size of 50~200μm.

3. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The flocculation pretreatment mentioned in step (1) refers to adding polyacrylamide flocculant with a concentration of 5~15 mg / L for flocculation pretreatment. After flocculation pretreatment, suspended solids are removed by cyclone classification or sedimentation.

4. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The alkaline precipitant mentioned in step (2) is selected from one or more of CaO, Ca(OH)2, CaCO3, K2CO3, KOH, NaOH, Na2CO3, and NaHCO3.

5. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The precipitation reaction in step (2) is carried out at a temperature of 20~90℃ for 5~60 min.

6. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The filtration process in step (2) to remove precipitates uses a filter screen with a pore size of 1~10μm.

7. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: In step (3), the Ca source is selected from one or more of CaCl2, Ca(OH)2, and CaO; the P source is selected from phosphoric acid; and the F source is selected from one or two of NaF and KF.

8. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The concentration of fluorapatite seed crystals added in step (4) is 0.2~1.0 wt%.

9. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The stirring crystallization in step (4) can be carried out by stirring crystallizer, continuous fluidized bed or circulating heat preservation crystallizer. The stirring crystallization temperature is 20~90℃ and the time is 0.5~24 h.

10. The method for directional recovery of fluorapatite from phosphogypsum leaching solution according to claim 1, characterized in that: The solid phase separated in step (4) is washed and dried to obtain Ca5(PO4)3F; the liquid phase is filtered through microfiltration, nanofiltration and reverse osmosis in sequence, and the effluent is directly recycled in the phosphogypsum washing section.

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