Method for recovering fluorapatite from ardealite leacheate through ion activity control

Abstract

The invention belongs to the technical field of ardealite resource utilization, and particularly relates to a method for recycling fluorapatite from ardealite leacheate through ion activity control. The method comprises the following steps: carrying out on-line detection on a fluorapatite saturation index on a phosphogypsum leacheate stock solution, and adjusting the supplementing amount of a Ca source / a P source / an F source in real time to keep a system supersaturated; adding background electrolyte, a complexing agent and a crystal face regulating agent into the treated solution to obtain a crystal precursor solution; controlling the temperature to be 50-90 DEG C, adjusting the pH value of the system to be 6.4-6.9, supplementing Ca < 2 + >, adding FA seed crystal, and uniformly stirring and mixing; the temperature is increased to 70-95 DEG C, the pH is increased to 6.8-7.8, a surface rearrangement accelerant is added for directional crystallization and solid-liquid separation, a solid product is washed and dried, and fluorapatite is obtained. FA crystal nucleation is controlled and regulated through ion activity, so that the yield and the purity of a target product are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of phosphogypsum resource utilization technology, specifically relating to a method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity. Background Technology

[0002] If the leaching solution undergoes multiple cycles or localized concentration during phosphogypsum treatment, PO4 will be produced. 3- =1-10 g / L, F - =0.5-5 g / L, Ca 2+ A high concentration system of 0.5-3 g / L is used. At this point, although the absolute content of impurity ions (Fe / Al / Si / Mg, etc.) remains unchanged, the relative concentration decreases, which weakens the interference with crystal nucleation and makes it possible to directly and directionally form fluorapatite (Ca5(PO4)3F, FA).

[0003] However, due to the high ionic strength, complex complexation reactions, and SO4 in high-concentration systems... 2- Strong competitive foundation, F - Fluorapatite readily induces CaF2 formation, making it difficult to reliably obtain high-purity fluorapatite using the traditional, simple "calcium supplementation-pH adjustment-crystallization" method. Common problems include: 1. Difficulty in separating CaF2 or mixed crystals (FA+CaF2); 2. Formation of hydroxyapatite (HAP) or carbonated hydroxyapatite, resulting in substandard P / F ratios in the product; 3. Irregular crystal morphology, large specific surface area, and numerous inclusions; 4. Easy accumulation of SO4 during mother liquor recycling. 2- Mg 2+ K + Na + This leads to seed passivation and a decrease in the crystallization rate. Summary of the Invention

[0004] In view of the shortcomings and deficiencies of the existing technology, the purpose of this invention is to provide a method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A method for recovering fluorapatite (FA) from phosphogypsum leachate by controlling ion activity includes the following steps:

[0007] (1) Characterization of the stock solution and dynamic formula calculation: The fluorapatite saturation index of the phosphogypsum leaching solution was detected online. The amount of Ca source, P source and F source added was adjusted in real time according to the fluorapatite saturation index to keep the system supersaturated.

[0008] (2) Ionic strength regulation and precursor construction: background electrolyte, complexing agent and crystal facet regulator are added to the solution after treatment in step (1) to obtain crystallization precursor solution;

[0009] (3) First stage of induced nucleation: control the temperature at 50-90℃, adjust the pH of the crystallization precursor solution system to 6.4-6.9, and add Ca 2+ Add FA seed crystals and stir to mix thoroughly;

[0010] (4) Second stage selective orientation growth: increase the temperature to 70-95℃ and increase the pH to 6.8-7.8, add surface rearrangement promoter to carry out directional crystallization, separate solid and liquid, and wash and dry the solid product to obtain fluorapatite (FA).

[0011] Furthermore, the PO4 content of the phosphogypsum leaching solution stock solution in step (1) is... 3- Concentration of 1-10 g / L, F - Concentration of 0.5-5 g / L, Ca 2+ The concentration is 0.5-3 g / L.

[0012] Further, the method for online detection of fluorapatite saturation index in step (1) is as follows: Ionic strength data of the original phosphogypsum leaching solution are collected, and the ionic strength is calculated using the formula I = 0.5ΣCi·Zi 2 Where Ci is the concentration of the i-th ion and Zi is its charge number; then the activity coefficient γi of each ion is calculated using the Davies equation or the extended Debye-Hückel equation, thus obtaining the ion activity ai = γi·Ci; the saturation index is calculated according to the fluorapatite dissolution equilibrium Ca5(PO4)3F → 5Ca 2+ +3PO4 3- +F - Calculate the ion activity product IAP of the system = (aCa 2+ ) 5 ·(aPO4 3- ) 3 ·(aF - The saturation index SI is obtained by comparing it with the dissolution equilibrium constant Ksp and obtaining SI = log(IAP / Ksp). The method for adjusting the amount of Ca, P and F sources added in real time to keep the system supersaturated is as follows: when SI < 0, the system is made supersaturated by adding Ca, P and / or F sources; when SI > 1, the effective supersaturation is reduced by adding alkali to adjust the pH value of the system to the range of 6 to 7 to avoid explosive precipitation.

[0013] The target ion activity value is obtained through the above calculations and converted into the required supplementary concentration ΔC. Then, the real-time supplementary amount is calculated based on the system volume V and the mass fraction ω of the effective component of the supplementary agent, so as to achieve dynamic and precise supplementary control of Ca source / P source / F source.

[0014] Furthermore, the background electrolyte mentioned in step (2) is one or both of NaNO3 and KCl, and the concentration of the background electrolyte is 0.05-0.5 mol / L.

[0015] The purpose of adding a background electrolyte in this invention is to adjust the Debye length of the solution, so that Ca 2+ -PO4 3- -F - It is easier to form precursor clusters locally, thereby improving the yield and purity of the target product, fluorapatite.

[0016] Further, the complexing agent mentioned in step (2) is one or more of soluble citrate, lactate, and EDTA, and the concentration of the complexing agent added is 10-100 mg / L.

[0017] The role of the complexing agent in this invention is to weakly complex Ca. 2+ This delays non-selective precipitation, improves the controllability of the nucleation window, and thus increases the yield and purity of the target product, fluorapatite.

[0018] Further, the crystal facet regulator mentioned in step (2) is one or two of low molecular weight (Mn≤5000) polyacrylic acid and phosphate oligomers, and the concentration of the crystal facet regulator is 10-100 mg / L.

[0019] The purpose of adding a crystal plane regulator in this invention is to promote the preferential growth of the (100) and (001) crystal planes, thereby improving the crystal morphology and crystal quality.

[0020] Furthermore, the Ca mentioned in step (3) 2+ The acceleration rate is 0.01-0.05 mol / L / h, and the amount of FA seed crystals added is 0.05-1.0 wt%.

[0021] By adding seed crystals and controlling Ca 2+ The acceleration rate is further adjusted to regulate the instantaneous supersaturation change rate of the system, thereby inducing the controllable generation of fluorapatite nuclei.

[0022] Furthermore, the pH adjustment rate during the process of adjusting the pH to 6.4-6.9 in step (3) and raising the pH to 6.8-7.8 in step (4) is ≤0.1 / min.

[0023] If the pH adjustment is performed too quickly in the above steps, it will result in: PO4 in the system3- With F - The effective concentration of the fluorapatite increases rapidly in a short period of time, causing the fluorapatite system to be in a highly supersaturated state instantaneously. This generates a large number of disordered microcrystalline nuclei, widens the crystal size distribution, and significantly refines the particles, resulting in poorer subsequent solid-liquid separation performance and increased moisture content in the filter cake.

[0024] Further, the surface rearrangement promoter mentioned in step (4) is one or more of sodium citrate and amino acids, and the concentration of the surface rearrangement promoter is 10-50 mg / L.

[0025] The surface rearrangement promoter molecule contains functional groups such as carboxyl and amino groups, which can react with Ca on the crystal surface. 2+ Reversible complexation occurs at the active sites, forming a weak adsorption layer at the crystal growth interface, thereby reducing the crystal surface energy and promoting the migration and rearrangement of atoms or ions between crystal planes.

[0026] Furthermore, the directional crystallization time in step (4) is 30-60 min, and the stirring intensity is reduced to 100-200 rpm during the directional crystallization process to form a weak shear zone to promote preferential growth.

[0027] Furthermore, the mixed system after directional crystallization described in step (4) is further subjected to the following third-stage polishing crystallization: removing part of the mother liquor, introducing a low ionic strength solution (a backwash solution or deionized water with a lower ionic strength than the mother liquor), and recrystallizing for 30-120 min.

[0028] The present invention employs a third-stage polishing and crystallization process, which allows impurities on the surface of the crystallized product to redissolve and FA to be redeposited, thereby further improving the purity of the target product.

[0029] Furthermore, the washing process in step (4) employs a three-stage countercurrent washing method: the first stage uses recycled water (high ionic strength) for washing, the second stage uses UF permeate (medium ionic strength) for washing, and the third stage uses pure water for gentle rinsing. This cascaded washing and low-loss desorption method reduces solvent loss.

[0030] Compared with the prior art, the beneficial effects of the present invention are:

[0031] (1) The concept of "ion activity window control" is proposed, which is achieved through multi-parameter coupling (pH-Ca activity-PO4 activity-F activity-SO4). 2- Suppressing the selective formation region of FA.

[0032] (2) By using interface modifiers / selective promoters (background electrolyte, complexing agent and crystal face modifier), the crystal nucleation energy barrier is controlled, the nucleation priority of FA is increased, thereby improving the yield and purity of the target product fluorapatite.

[0033] (3) Crystal purity and uniformity are ensured by using a hierarchical crystallization system (first stage induced nucleation, second stage selective orientation growth, and third stage polishing crystallization). Attached Figure Description

[0034] Figure 1 This is a general process flow diagram of a method for recovering fluorapatite (FA) from phosphogypsum leachate by controlling ion activity, as described in Example 1.

[0035] Figure 2 The image shows the XRD pattern of the FA product obtained in Example 1.

[0036] Figure 3 The image shows the SEM image of the FA product obtained in Example 1. Detailed Implementation

[0037] 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.

[0038] Example 1

[0039] A method for recovering fluorapatite (FA) from phosphogypsum leachate by controlling ion activity is illustrated in the following process flow diagram: Figure 1 As shown, it includes the following steps:

[0040] (1) Characterization of the raw solution and dynamic formula calculation: The raw solution of phosphogypsum leaching solution (2.00 L) was tested online (PO4 content was 0.00 L). 3- (As PO4) = 4.00 g / L, F - =0.78 g / L, Ca 2+ =3.00 g / L, Mg 2+ =0.50 g / L, SO4 2- =12g / L, initial pH≈2.6), the activity of each ion was calculated based on the online detection values, and the ionic strength of the system was calculated according to the formula I=0.5ΣCi·Zi 2 Calculate, where PO4 3- F - Ca 2+ The molar concentrations were 0.0421 mol / L, 0.0411 mol / L, and 0.0750 mol / L, respectively, corresponding to an ionic strength of approximately I = 0.36. Under these conditions, the ionic activity coefficient was calculated using the Davies activity model, yielding γCa. 2+ ≈0.286, γPO4 3- ≈0.060, γF - ≈0.73, thus the ion activity aCa was calculated. 2+ ≈0.0215, aPO4 3- ≈0.00253, aF- ≈0.0300; based on the dissolution equilibrium of fluorapatite Ca5(PO4)3F→5Ca 2+ +3PO4 3- +F⁻ Calculate the ion activity product IAP of the system = (aCa 2+ ) 5 ·(aPO4 3- ) 3 ·(aF - Its value is approximately 1.5 × 10⁻⁶. -18 Compared with the dissolution equilibrium constant Ksp of fluorapatite at 25℃ (logKsp≈−57), the saturation index SI=log(IAP / Ksp) is significantly greater than 1, indicating that the system is in a highly supersaturated state. Therefore, the model judges that there is no need to add an additional Ca or P source, but to reduce the effective supersaturation by adjusting the alkalinity. The actual control strategy is not to add CaCl2·2H2O, but to adjust and stabilize the pH of the system in the range of 6.5~6.8 by slowly adding NaOH, so as to achieve controllable induced nucleation of fluorapatite.

[0041] (2) Ionic strength regulation and precursor construction: 0.20 mol / L NaNO3 was added to the solution after treatment in step (1) as a background electrolyte, and 50 mg / L sodium citrate complexing agent was added to weakly complex Ca. 2+ (Delayed non-selective precipitation), and 50 mg / L of low molecular weight polyacrylic acid (PAA, Mn≈2000) was added as a crystal facet modifier to obtain a crystallization precursor solution.

[0042] (3) First stage of induced nucleation: control the temperature at 65℃, adjust the pH of the crystallization precursor solution system to 6.6 (pH adjustment rate ≤ 0.1 pH / min), and add Ca. 2+ The rate was set to 0.03 mol / L / h, and 0.5 wt% FA seed crystals were added. The mixture was stirred thoroughly and homogenized while maintaining a moderate shear rate (500 rpm).

[0043] (4) Second stage selective orientation growth: Increase the temperature to 85℃ and slowly increase the pH to 7.2 (pH change rate ~0.05 / min), reduce the stirring intensity (200 rpm) to form a weak shear zone to promote preferential growth, and add 20 mg / L sodium citrate as a surface rearrangement promoter for directional crystallization for 45 min.

[0044] (5) Third stage polishing and crystallization: In order to further improve the purity, 20% of the mother liquor was removed and deionized water was used to replenish the liquid to reduce the ionic strength of the system. Recrystallization was carried out for 60 min to dissolve soluble impurities on the surface and redeposit purified crystals.

[0045] (6) Centrifugation solid-liquid separation. The solid product is washed in three stages of countercurrent washing. The first stage uses recycled water (high ionic strength to reduce solubility); the second stage uses UF permeate (medium ionic strength); and the third stage uses pure water for light washing (to remove residual soluble salts). After drying, 13.10 g of fluorapatite (FA) is obtained.

[0046] In this embodiment, the calculated residual PO4 in the mother liquor is 0.30 g / L × 2.00 L = 0.60 g, and the residual F... - =0.03 g / L × 2.00 L = 0.06 g, initial total PO4 = 8.00 g, recovered PO4 mass = 7.40 g, PO4 recovery rate = 7.40 ÷ 8.00 × 100% = 92.5%. The theoretical initial molar number of PO4 is 0.08422 mol, the theoretical molar number of FA produced is 0.028073 mol, the theoretical FA mass is 14.15 g, and the measured FA product yield = 13.10 g / 14.15 g × 100% = 92.6%, which is very consistent with the PO4 recovery rate of 92.5%, indicating that the product is mainly FA phase, and the loss mainly comes from mother liquor residue and washing dissolution loss.

[0047] The XRD pattern of the obtained FA product is as follows Figure 2 As shown, the main peak of the XRD pattern perfectly matches the peak position of the standard Ca5(PO4)3F, with no obvious CaF2 or Ca3(PO4)2 peaks, indicating that the FA has high purity and good crystal form.

[0048] The SEM images of the obtained FA products are as follows: Figure 3 As shown in the figure. SEM images show that the FA crystals are elongated and have a distinct crystal shape.

[0049] The ICP-OES test results of the obtained FA products are shown in Table 1 below:

[0050] Table 1. ICP-OES characterization results of FA products

[0051] element P <![CDATA[P2O5]]> Mg Al Fe Si FA(%) 18.21 41.73 0.14 0.31 0.26 0.33

[0052] ICP-OES testing showed that the P content was 18.21%, which translates to a P2O5 content of 41.73%, and the total impurities (based on soluble metals) were less than 2 wt% (the sum of Mg, Fe, Al, and Si is <2 wt%).

[0053] Example 2

[0054] A method for recovering fluorapatite (FA) from phosphogypsum leachate by controlling ion activity includes the following steps:

[0055] (1) The characterization of the stock solution and the dynamic formulation calculation are the same as in Example 1.

[0056] (2) Ionic strength regulation and precursor construction: 0.30 mol / L KCl was added to the solution after treatment in step (1) as a background electrolyte, and 10 mg / L EDTA complexing agent was added to weakly complex Ca. 2+ (Delayed non-selective precipitation), and 20 mg / L of low molecular weight polyacrylic acid (PAA, Mn≈2000) was added as a crystal facet modifier to obtain a crystallization precursor solution.

[0057] (3) First stage of induced nucleation: control the temperature at 75℃, adjust the pH of the crystallization precursor solution system to 6.5 (pH adjustment rate ≤ 0.1 pH / min), and add Ca. 2+ The rate was set to 0.04 mol / L / h, and 0.3 wt% FA seed crystals were added. The mixture was stirred thoroughly and homogeneously while maintaining a moderate shear rate (500 rpm).

[0058] (4) Second stage selective orientation growth: Increase the temperature to 90℃ and slowly increase the pH to 7.5 (pH change rate ~0.05 / min), reduce the stirring intensity (200 rpm) to form a weak shear zone to promote preferential growth, and add 50 mg / L sodium citrate as a surface rearrangement promoter for directional crystallization for 30 min.

[0059] (5) Third stage polishing and crystallization: In order to further improve the purity, 25% of the mother liquor was removed and deionized water was used to replenish the liquid to reduce the ionic strength of the system. Recrystallization was carried out for 90 min to dissolve soluble impurities on the surface and redeposit purified crystals.

[0060] (6) Centrifugation was used for solid-liquid separation. The solid product was washed in three stages of countercurrent washing. The first stage used recycled water (high ionic strength to reduce solubility); the second stage used UF permeate (medium ionic strength); and the third stage used pure water for light washing (to remove residual soluble salts). After drying, 13.16 g of fluorapatite (FA) was obtained. The yield of FA product was 93.0%.

[0061] Example 3

[0062] A method for recovering fluorapatite (FA) from phosphogypsum leachate by controlling ion activity includes the following steps:

[0063] (1) The characterization of the stock solution and the dynamic formulation calculation are the same as in Example 1.

[0064] (2) Ionic strength regulation and precursor construction: 0.10 mol / L NaNO3 was added to the solution after treatment in step (1) as a background electrolyte, and 100 mg / L sodium lactate complexing agent was added to weakly complex Ca. 2+ (Delayed non-selective precipitation), and 100 mg / L sodium tripolyphosphate was added as a crystal facet modifier to obtain a crystallization precursor solution.

[0065] (3) First stage of induced nucleation: control the temperature at 85℃, adjust the pH of the crystallization precursor solution system to 6.9 (pH adjustment rate ≤ 0.1 pH / min), and add Ca. 2+ The rate was set to 0.05 mol / L / h, and 0.1 wt% FA seed crystals were added. The mixture was stirred thoroughly and homogenized while maintaining a moderate shear rate (500 rpm).

[0066] (4) Second stage selective orientation growth: Increase the temperature to 95℃ and slowly increase the pH to 7.8 (pH change rate ~0.05 / min), reduce the stirring intensity (200 rpm) to form a weak shear zone to promote preferential growth, and add 10 mg / L of lysine as a surface rearrangement promoter for directional crystallization for 60 min.

[0067] (5) Third stage polishing and crystallization: In order to further improve the purity, 30% of the mother liquor was removed and deionized water was used to replenish the liquid to reduce the ionic strength of the system. Recrystallization was carried out for 120 min to dissolve the soluble impurities on the surface and redeposit the purified crystals.

[0068] (6) Centrifugation was used for solid-liquid separation. The solid product was washed in three stages of countercurrent washing. The first stage used recycled water (high ionic strength to reduce solubility); the second stage used UF permeate (medium ionic strength); and the third stage used pure water for light washing (to remove residual soluble salts). After drying, 12.98 g of fluorapatite (FA) was obtained. The yield of FA product was 91.7%.

[0069] Comparative Example 1

[0070] Compared with Example 1, this comparative example is the same except that NaNO3 was not added as a background electrolyte in step (2) ion strength regulation and precursor construction.

[0071] The ionic strength of this comparative system fluctuated from approximately 0.55 in Example 1 to 0.32–0.40, with the ionic activity coefficient changing by more than 30% during the reaction, leading to a significant increase in localized instantaneous supersaturation. Experimental results indicate that residual PO4 in the mother liquor… 3- The concentration was increased to 0.85 g / L, F - Increasing the concentration to 0.12 g / L corresponds to a PO4 recovery rate of approximately 78.4%, with the resulting solid containing approximately 77% to 80% FA. At the same time, a significant weak peak of CaF2 appears, and the average particle size decreases from approximately 8.5 μm in Example 1 to 4.2 μm, resulting in a significant deterioration in the product's filtration performance.

[0072] Comparative Example 2

[0073] Compared with Example 1, this comparative example is identical except that sodium citrate complexing agent was not added in step (2) of ion strength regulation and precursor construction.

[0074] This comparative example of free Ca 2+ The activity increased instantaneously by about 40% during the pH rise from 6.0 to 6.6, leading to the preferential formation of CaF2 and amorphous calcium phosphate. Experimental results showed that approximately 12% of the F2 activity had been generated after 30 minutes of reaction. - It precipitates as CaF2, and the final mother liquor contains residual PO4. 3- 0.72 g / L, F - With a concentration of 0.15 g / L, the PO4 recovery rate was only about 82.0%, the theoretical purity of FA in the product decreased to about 80%~82%, and even with a longer recrystallization time, it was still difficult to completely eliminate the impurity phase.

[0075] Comparative Example 3

[0076] Compared with Example 1, this comparative example is the same except that low molecular weight polyacrylic acid was not added as a crystal plane regulator in step (2) of ion strength regulation and precursor construction.

[0077] In this comparative example, the anisotropic growth of FA crystals was suppressed, the length and diameter of the FA crystals decreased compared to Example 1, the aggregation between crystals was significantly enhanced, and the residual PO4 in the mother liquor after polishing and crystallization was reduced. 3- The concentration remained at 0.48 g / L, and the final FA yield was approximately 12.2 g, corresponding to a yield of 86.2%, which was significantly lower than the 92.6% in Example 1.

[0078] Comparative Example 4

[0079] Compared with Example 1, this comparative example did not add sodium citrate as a surface rearrangement promoter during the second stage of selective orientation growth in step (4), but the rest were the same.

[0080] The surface defects of the crystals in this comparative example are difficult to eliminate through the dissolution-redeposition process, resulting in a significant increase in the surface roughness of the FA crystals and an increase in the full width at half maximum (FWHM) of the XRD peaks, indicating a decrease in crystallinity. Simultaneously, impurity entrainment reduces the effective purity of FA to approximately 83%–85%, and residual PO4 in the mother liquor further contributes to the problem. 3- The concentration was increased to 0.52 g / L, and the final measured yield of FA was 12.5 g, corresponding to approximately 88.3%, indicating that the surface rearrangement promoter has a significant effect on improving crystal integrity and recovery efficiency.

[0081] 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 recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity, characterized in that... Includes the following steps: (1) Characterization of the stock solution and dynamic formula calculation: The fluorapatite saturation index of the phosphogypsum leaching solution was detected online. The amount of Ca source, P source and F source added was adjusted in real time according to the fluorapatite saturation index to keep the system supersaturated. (2) Ionic strength regulation and precursor construction: background electrolyte, complexing agent and crystal facet regulator are added to the solution after treatment in step (1) to obtain crystallization precursor solution; (3) First stage of induced nucleation: control the temperature at 50-90℃, adjust the pH of the crystallization precursor solution system to 6.4-6.9, and add Ca 2+ Add FA seed crystals and stir to mix thoroughly; (4) Second stage selective orientation growth: increase the temperature to 70-95℃ and increase the pH to 6.8-7.8, add surface rearrangement promoter to carry out directional crystallization, separate solid and liquid, and wash and dry the solid product to obtain fluorapatite.

2. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The PO4 content of the phosphogypsum leaching solution in step (1) 3- Concentration of 1-10 g / L, F - Concentration of 0.5-5 g / L, Ca 2+ The concentration is 0.5-3 g / L.

3. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The method for online detection of fluorapatite saturation index in step (1) is as follows: collect the ionic strength data of the original phosphogypsum leaching solution, and the ionic strength calculation formula is I=0.5ΣCi·Zi 2 Where Ci is the concentration of the i-th ion and Zi is its charge number; then the activity coefficient γi of each ion is calculated using the Davies equation or the extended Debye-Hückel equation, thus obtaining the ion activity ai = γi·Ci; the saturation index is calculated according to the fluorapatite dissolution equilibrium Ca5(PO4)3F → 5Ca 2+ +3PO4 3- +F - Calculate the ion activity product IAP of the system = (aCa 2+ ) 5 ·(aPO4 3- ) 3 ·(aF - The saturation index SI is obtained by comparing it with the dissolution equilibrium constant Ksp and obtaining SI = log(IAP / Ksp). The method for adjusting the amount of Ca, P and F sources added in real time to keep the system supersaturated is as follows: when SI < 0, the system is made supersaturated by adding Ca, P and / or F sources; when SI > 1, the effective supersaturation is reduced by adding alkali to adjust the pH value of the system to the range of 6 to 7 to avoid explosive precipitation.

4. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The background electrolyte in step (2) is one or two of NaNO3 and KCl, and the concentration of the background electrolyte is 0.05-0.5 mol / L; the complexing agent is one or more of soluble citrate, lactate, and EDTA, and the concentration of the complexing agent is 10-100 mg / L; the crystal facet modifier is one or two of low molecular weight polyacrylic acid with Mn≤5000 and phosphate oligomer, and the concentration of the crystal facet modifier is 10-100 mg / L.

5. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The Ca mentioned in step (3) 2+ The acceleration rate is 0.01-0.05 mol / L / h, and the amount of FA seed crystals added is 0.05-1.0 wt%.

6. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: During the process of adjusting the pH to 6.4-6.9 in step (3) and raising the pH to 6.8-7.8 in step (4), the pH adjustment rate is ≤0.1 / min.

7. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The surface rearrangement promoter mentioned in step (4) is one or more of sodium citrate and amino acids, and the concentration of the surface rearrangement promoter is 10-50 mg / L.

8. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The directional crystallization time in step (4) is 30-60 min. During the directional crystallization process, the stirring intensity is reduced to 100-200 rpm to form a weak shear zone to promote preferential growth.

9. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The mixed system after directional crystallization described in step (4) is further subjected to the following third stage of polishing and crystallization: removing part of the mother liquor and introducing a low ionic strength solution for recrystallization for 30-120 min.

10. The method for recovering fluorapatite from phosphogypsum leaching solution by controlling ion activity according to claim 1, characterized in that: The washing process described in step (4) employs a three-stage countercurrent washing method: the first stage uses recycled water for washing, the second stage uses UF permeate for washing, and the third stage uses pure water for gentle washing.

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