Method for leaching and separating phosphorus from modified sludge incineration ash and preparing FePO4

Abstract

The invention discloses a method for leaching and separating phosphorus from modified sludge incineration ash and preparing FePO4, which comprises the following steps: firstly, mixing a modifier with sludge incineration ash ISSA, modifying at high temperature, leaching with dilute acid, and separating to obtain a phosphorus-containing leachate and residues; then adding an iron-containing substance into the phosphorus-containing leachate according to a Fe / P molar ratio of (1-1.2): 1, adjusting the pH value of a reaction system to 1.0-4.0, and carrying out chemical precipitation reaction by controlling the temperature within the range of 40-90 DEG C to generate FePO4 hydrate precipitate; finally, the precipitate is dried and calcined, a high-purity FePO4 product is obtained, the content of FePO4 in the product exceeds 93%, and the product can be used as a raw material for preparing batteries. The method achieves the phosphorus dissolution rate of 95% or above in the ISSA, has the advantages of being simple in process, low in energy consumption and cost, environmentally friendly and the like, achieves efficient resource utilization and phosphorus resource recovery of the ISSA, and is suitable for large-scale application.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid waste resource utilization, and particularly relates to a method for leaching and separating phosphorus from modified sludge incineration ash and preparing FePO4. Background Art

[0002] With the acceleration of the urbanization process and the development of sewage treatment technology, the sludge output increases year by year. The phosphorus content in the incinerated sludge ash (ISSA) produced by incineration treatment reaches 11% - 20%, which is a secondary phosphorus resource with great development value. However, the heavy metal (Zn, Cu, Pb, etc.) content in ISSA exceeds the standard and P mainly exists in the form of Al / Fe-phosphate that is difficult for plants to absorb. Direct land composting will increase environmental pollution and ecological risks. Therefore, the safe treatment of ISSA and the P recovery technology have become the research focus. As a cathode material for lithium-ion batteries, iron phosphate has been widely used due to its excellent properties such as high safety, long cycle life, and low cost. FePO4 is the key precursor for preparing lithium iron phosphate. At home and abroad, ferrous sulfate is generally used to react with phosphoric acid or phosphate to prepare pure FePO4, which highly depends on limited phosphate rock resources, resulting in its supply stability and cost being easily affected by fluctuations in the resource market. With the rapid development of China's new energy industry, the demand for FePO4 continues to grow and the price keeps rising. In order to reduce the manufacturing cost of new energy batteries and build a resource recycling society, it is possible to consider separating and recovering phosphorus resources from solid waste and preparing FePO4 materials. Therefore, developing a technical method that can efficiently and selectively extract phosphorus from ISSA, effectively separate it from other elements, and finally obtain high-purity FePO4 has become a research hotspot in the current resource recovery field.

[0003] Hydrometallurgy is currently the main method for recovering phosphorus resources from ISSA. Although it can achieve a phosphorus recovery rate of 80% - 95%, there are still obvious limitations. This method requires a large amount of strong acid, which not only significantly increases the treatment cost but also causes serious equipment corrosion. Most importantly, the traditional leaching process will lead to the synchronous dissolution of Al, Fe, Si, and heavy metal elements (such as Zn, Cu, Pb), increasing the difficulty and cost of subsequent phosphorus separation and purification. Existing research mainly ensures the purity of the final phosphorus product by deeply purifying the acidic leaching solution. For example, Chinese Patent CN118929605A uses the combined technology of CaCl2 activation - electrochemical separation to achieve the concentration of the leaching solution and the selective separation of phosphorus anions, preparing a slow - release phosphate fertilizer product with high purity and low pollution. Chinese Patent CN117401657A has developed an EDTA - assisted hydrothermal - lanthanide adsorbent adsorption process, with a final phosphorus recovery rate of over 98%. Chinese Patent CN113401887B proposes a method for recovering FePO4 precipitate from ISSA leaching solution, achieving a phosphorus leaching rate of 97%. However, the leaching solutions of the above patents contain a large number of impurity ions and need to be purified through multiple steps to obtain a pure phosphate solution. The process is long and the cost is high. During the pH adjustment for impurity removal, Fe 3+ / Al 3+ also forms a precipitate with PO4 3- , resulting in a significant reduction in phosphorus recovery rate. Similarly, the method for recovering FePO4 from ISSA leaching solution in Chinese Patent CN116143094A also has technical defects such as low phosphorus recovery rate and the need for multi - stage recycling of the leaching solution. In summary, traditional acid leaching generally has problems such as high energy consumption, high reagent cost, and serious phosphorus loss, and the process flow is complex, leading to a high treatment cost, which severely restricts the industrial application of ISSA phosphorus recovery technology. There is an urgent need to develop a more efficient and economical new phosphorus extraction process. Summary of the Invention

[0004] In order to overcome the above - mentioned defects existing in the prior art, the purpose of the present invention is to provide a process for efficiently and low - cost leaching, separating phosphorus, and preparing FePO4 from incinerated sewage sludge ash (ISSA). First, by performing high - temperature modification treatment on ISSA, the leaching efficiency of P under acidic conditions is improved, and at the same time, the concentrations of Al, Fe, Si, and heavy metal ions in the leaching solution are reduced. Then, without additional purification treatment of the leaching solution, only by adjusting parameters such as the Fe / P molar ratio, reaction pH, and reaction time, high - purity FePO4 products can be directly obtained through chemical precipitation. This method realizes the efficient recovery and high - value utilization of phosphorus resources in ISSA, and at the same time has significant advantages in process simplification and cost.

[0005] To achieve the above-mentioned invention object, the present invention provides a method for leaching and separating phosphorus from reformed sludge incineration ash and preparing FePO4, and the method comprises the following steps:

[0006] ① Mix the sludge incineration ash (ISSA) with a modifier and then conduct high-temperature modification treatment, leach with dilute acid, and separate to obtain a phosphorus-containing leachate and a residue; the residue can be used as a road construction material.

[0007] ② Add an iron-containing substance (iron source) to the leachate according to an Fe / P molar ratio of 1-1.2:1, and adjust the pH of the reaction system to 1.0-4.0;

[0008] ③ Control the temperature within the range of 40°C - 90°C to conduct a chemical precipitation reaction to generate a FePO4 hydrate precipitate; perform solid-liquid separation to obtain the FePO4 hydrate precipitate.

[0009] ④ The FePO4 hydrate precipitate is dried and calcined to obtain a high-purity FePO4 product; this product can be used as a battery material.

[0010] In the above technical solution, further, the modifier for the high-temperature modification treatment in step ① is selected as CaO, and the addition amount of CaO is 10% - 40% of the mass of the sludge incineration ash; the temperature of the high-temperature modification treatment is 800°C - 1100°C. The dilute acid solution used in the acid leaching process in step ① is dilute HCl, dilute HNO3 or dilute H2SO4 solution, and the concentration of the dilute acid solution is 0.5 - 2.5 mol / L; during the acid leaching process, the pH value of the leachate is stabilized at 1.0 - 3.5, and the leaching reaction lasts for 30 - 80 min.

[0011] Adding a modifier (such as CaO / MgO) promotes the conversion of non-apatite phosphorus (NAIP, Al / Fe-phosphate) in ISSA into acid-soluble apatite phosphorus (AP, Ca / Mg-l phosphate) under high-temperature conditions, and Al, Fe and other heavy metal elements react with SiO2 to form a stable silicate phase, which is almost insoluble in dilute acid, effectively inhibiting the dissolution of impurity elements and realizing the effective separation of P and impurity elements.

[0012] In an acidic environment with a pH of 1.0 - 3.5, the main phosphorus-containing phase (Ca / Mg-phosphate) in the reformed ISSA reacts with H + resulting in the dissolution and release of Ca 2+ and Mg 2+ and their conversion into soluble calcium salts and magnesium salts (as shown in chemical reaction formulas R1 - R4). During the leaching process, H +The consumption of will lead to an increase in the pH of the system, which may cause precipitation of phosphates such as FePO4 and AlPO4. By automatically regulating the pH to maintain the optimal leaching conditions, not only can the efficient dissolution of Ca / Mg-phosphates be ensured, but also the precipitation of impurities can be inhibited, creating conditions for the subsequent recovery of high-purity FePO4.

[0013] Ca3(PO4)2 + 6H + =3Ca 2+ + 2H3PO4 (R1)

[0014] Ca3(PO4)2 + 4H + =3Ca 2+ + 2H2PO4 - (R2)

[0015] Mg3(PO4)2 + 6H + =3Mg 2+ + 2H3PO4 (R3)

[0016] Mg3(PO4)2 + 4H + =3Mg 2+ + 2H2PO4 - (R4)

[0017] Furthermore, the iron-containing substance described in step ② is selected from at least one of FeCl3, Fe(NO3)3 or Fe2(SO4)3; the pH of the system is adjusted by adding an alkaline solution in step ②, and the alkaline solution is selected from at least one of NaOH solution, Ca(OH)2 suspension or ammonia water, and the concentration of the alkaline solution is 5 - 15 mol / L.

[0018] After leaching, the initial pH value of the ISSA leaching solution is relatively low, and the contents of Fe, Al, Si and heavy metals are relatively small. Therefore, no additional purification and impurity removal steps are required. Only by adding an iron-containing substance according to the Fe / P molar ratio and regulating the pH within the range of 1.0 - 4.0 can the process requirements for FePO4 precipitation be met.

[0019] Furthermore, during the chemical precipitation reaction process of step ③, the reaction time is 0.5 - 2.0 h, and the stirring rate is controlled within 30 - 300 r / min.

[0020] Furthermore, the drying conditions of step ④ are: drying temperature 60°C - 110°C, drying time 0.5 - 4.0 h; the calcination treatment conditions of step ④ are: calcination temperature 300°C - 700°C, calcination time 0.5 - 2.0 h.

[0021] The above method is easy to operate, significantly reducing the alkali consumption. At the same time, it utilizes iron-containing raw materials that are cheap and easily available, possessing an obvious cost advantage. In the field of phosphorus resource recovery, this technical solution not only has high feasibility in implementation but also has important promotion and application value, providing an efficient solution for the recycling of phosphorus.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] ① After the high-temperature modification treatment of the present invention, the Al / Fe-phosphate in ISSA is successfully converted into acid-soluble apatite phosphorus Ca / Mg-phosphate, significantly improving the leaching efficiency of phosphorus and achieving a phosphorus dissolution rate of more than 95% in ISSA.

[0024] ② The Al, Fe, Si, and heavy metal elements in the modified ISSA exist stably in the form of oxides or silicates and are hardly dissolved during leaching under dilute acid conditions, effectively avoiding the problem of the dissolution of impurity elements in the traditional acid leaching process and significantly reducing the risk of secondary pollution.

[0025] ③ The present invention innovatively omits the steps of purifying and removing impurities from the leaching solution in the traditional leaching process (such as electrochemical separation, adsorbent treatment, etc.). By directly controlling the Fe / P molar ratio, an iron-containing substance is added to the leaching solution, and FePO4 products are obtained in one step by chemical precipitation method, significantly reducing the production cost.

[0026] ④ The initial pH of the leaching solution is appropriate, and only a fine adjustment of the pH value is required to meet the reaction conditions, greatly reducing the dosage of alkaline substances. The operation is simple and the cost is low, making it suitable for large-scale application.

[0027] ⑤ By controlling the Fe / P molar ratio, reaction pH value, and reaction time, FePO4 products with high purity can be obtained, which can be used as raw materials for preparing batteries to meet the demand for FePO4 in the new energy industry.

[0028] ⑥ The process conditions in the present invention are mild, the equipment requirements are simple, and it is easy to realize industrial production; this high-efficiency and low-cost phosphorus recovery technology is not only applicable to the resource utilization of ISSA but also can be extended to the treatment of phosphorus-containing solid wastes such as phosphorus chemical industry waste residues and livestock and poultry manure incineration ash, having a broad market application prospect. Description of the Drawings

[0029] Figure 1 It is a process flow diagram for leaching and separating phosphorus from ISSA and preparing FePO4 according to the present invention;

[0030] Figure 2 It is a bar chart of the dissolution rates of the main elements in ISSA before and after modification in Example 1;

[0031] Figure 3Photograph of the FePO4 product recovered from the modified ISSA leachate in Example 1;

[0032] Figure 4 XRD pattern of the FePO4 product prepared in Example 1. Detailed implementation manners

[0033] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited in any way. To avoid redundancy, in the following embodiments, raw materials are commercially available products unless otherwise specified, and methods used are conventional methods unless otherwise specified.

[0034] A method for leaching and separating phosphorus from modified sludge incineration ash and preparing FePO4, as Figure 1 shown in the process flow chart, the method includes the following steps:

[0035] ① Mix the sludge incineration ash with a modifier and perform high-temperature modification treatment, leach with dilute acid, and separate to obtain a phosphorus-containing leachate and a residue;

[0036] ② Add an iron-containing substance to the leachate according to an Fe / P molar ratio of 1-1.2:1, and adjust the pH of the reaction system to 1.0-4.0;

[0037] ③ Control the temperature within the range of 40°C to 90°C to carry out a chemical precipitation reaction to form a FePO4 hydrate precipitate;

[0038] ④ The precipitate is dried and calcined to obtain a high-purity FePO4 product.

[0039] For the parts not described in the following embodiments, they are the same as the description content of the above specific implementation manners.

[0040] Example 1

[0041] A method for leaching and separating phosphorus from modified sludge incineration ash and preparing FePO4, the method includes the following steps:

[0042] ① Mix the sludge incineration ash ISSA with a modifier and perform high-temperature modification treatment, leach with dilute acid, and separate to obtain a phosphorus-containing leachate and a residue; The modifier used for the high-temperature modification treatment is CaO, and the addition amount of CaO is 20% of the mass of the sludge incineration ash; The temperature of the high-temperature modification treatment is 1000°C; The dilute acid solution used in the acid leaching process is a dilute HCl solution with a concentration of 2.0 mol / L; During the acid leaching process, the pH value of the leachate is stabilized at 1.0, and the leaching reaction lasts for 30 min.

[0043] The chemical compositions of ISSA before and after modification were analyzed using an X-ray fluorescence analyzer (XRF). The results are shown in Table 1. The main elemental components in ISSA are Si, P, Ca, Al, and Fe, among which the P content is as high as 7.27%, which can be used as a secondary phosphorus source. After high-temperature modification with CaO, the Ca content in ISSA increased to 14.77%, and the Ca / P molar ratio increased to 1.65.

[0044] Table 1 Chemical Compositions of ISSA before and after Modification in Example 1 (Mass Fraction / %)

[0045]

[0046] In unmodified ISSA, phosphorus mainly exists in the Mg3Ca3(PO4)4 phase, and a small amount exists in the form of Fe / Al-phosphate. After high-temperature modification with CaO, Ca 2+ replaces the metal cations in Mg / Al / Fe-phosphate through cation exchange, promoting P to mainly exist in the acid-soluble (Ca 2.589 Mg 0.411 )(PO4)2 mineral phase, creating favorable conditions for the subsequent selective leaching of P.

[0047] ISSA was placed in a dilute acid solution for leaching, and an automatic pH control system was used to maintain the pH value of the leaching solution stable at 1.0. After the leaching reaction lasted for 30 min, solid-liquid separation was achieved by vacuum filtration to obtain a phosphorus-containing leaching solution and acid leaching residue. An inductively coupled plasma atomic emission spectrometer (ICP-OES) was used to measure the mass concentration of each element in the leaching solution, and the dissolution rate of each element in the acid solution before and after the modification of the incineration ash was calculated using the following formula.

[0048]

[0049] In formula (1), R M is the dissolution rate of element M in the leaching solution; C M is the mass concentration of element M in the leaching solution, mg / L; V is the volume of the final solution, L; m M is the mass of element M in the sludge incineration ash, g.

[0050] The dissolution rates (R M ) of the main elements in ISSA before and after modification in Example 1 are as shown in Figure 2As shown, the results show that after leaching for 30 min under acidic conditions with pH = 1.0, the P dissolution rate in the unmodified ISSA is relatively low (54.33%), while the dissolution rates of Ca, Mg, and Mn are relatively high, reaching 78.34%, 69.84%, and 86.77% respectively. After the modification treatment, the P dissolution rate in ISSA is significantly increased to 97.39%, indicating that the modification treatment can effectively promote the release of phosphorus. In addition, the leaching rates of Ca, Mg, and Mn all show a downward trend, and the leaching rates of Fe, Al, and Si still remain at a relatively low level (less than 5%). This result shows that the CaO modification treatment not only significantly improves the selective leaching efficiency of phosphorus, but also inhibits the leaching of impurity elements through phase reconstruction, providing favorable conditions for the efficient recovery of phosphorus.

[0051] ② Add iron-containing substances to the leaching solution according to the Fe / P molar ratio of 1:1, and adjust the pH of the reaction system to 1.0; the iron-containing substance is a 0.5 mol / L FeCl3 solution, and the pH of the system is adjusted by adding an alkaline solution, and the alkaline solution is selected from NaOH solution with a concentration of 10 mol / L.

[0052] The chemical precipitation method is used to recover phosphorus from the leaching solution. After the modified ISSA is selectively leached with dilute hydrochloric acid at pH = 1.0, the obtained leaching solution is acidic, and the phosphorus mass concentration is as high as 576 mg / L, providing an ideal phosphorus source for the subsequent recovery of FePO4.

[0053] ③ Control the temperature within 80 °C for the chemical precipitation reaction to generate FePO4 hydrate precipitate; the chemical precipitation reaction time is 1.0 h, and the stirring rate is controlled at 200 r / min.

[0054] Place the mixed solution in a constant temperature water bath at 80 °C for 1 h to promote the formation of FePO4 precipitate; after the reaction, perform solid-liquid separation to obtain FePO4 hydrate precipitate; use ICP-OES to measure the concentrations of various elements in the supernatant after separating the precipitate, and the detection results are shown in Table 2.

[0055] Table 2 Concentrations of various elements in the ISSA leaching solution and the supernatant after separating the precipitate in Example 1 (mg / L)

[0056]

[0057] ④ Dry the hydrate precipitate obtained in step ③ in an oven at 80 °C for 2 h, and then calcine it in a muffle furnace at 600 °C for 1.5 h to obtain a high-purity crystalline FePO4 product. Use XRF to analyze the chemical composition of the product in Example 1, and the measurement results of the mass content of the chemical composition are listed in Table 3.

[0058] After adding FeCl3 solution to the leaching solution, the Fe mass concentration reached 940.18 mg / L (Fe / P molar ratio = 1.0); after the precipitation reaction ended, the residual concentrations of P and Fe in the supernatant were significantly reduced, and their precipitation rates were as high as 97.18% and 99.23% respectively. The concentrations of most Mg and Si in the solution were relatively high and remained in a dissolved state. The XRF analysis results (Table 3) showed that the precipitation product mainly consisted of Fe2O3 (47.50%) and P2O5 (50.31%), with a total content of 97.47%, and the impurity content was relatively low (<3%). The photograph of the precipitation product prepared in Example 1 is as Figure 3 shown and is a light yellow powder. The X-ray diffraction (XRD) analysis results of the precipitation product prepared in Example 1 showed ( Figure 4 ), the characteristic peaks of the precipitate matched those of the standard Fe(PO4) (JCPDS No. 72-2124), and no other impurity phase peaks were detected, indicating that the product had good crystallinity and high chemical purity and could be used as a raw material for preparing batteries.

[0059] Example 2

[0060] A method for leaching and separating phosphorus from modified sludge incineration ash and preparing FePO4, the method comprising the following steps:

[0061] ① Mix the sludge incineration ash ISSA with a modifier and perform high-temperature modification treatment, then leach with dilute acid to separate a phosphorus-containing leaching solution and a residue; the modifier for the high-temperature modification treatment is CaO, and the addition amount of CaO is 10% of the mass of the sludge incineration ash; the temperature of the high-temperature modification treatment is 800 °C and the treatment time is 2 h. The dilute acid solution used in the acid leaching process is a dilute H2SO4 solution, and the concentration of the dilute acid solution is 0.5 mol / L; during the acid leaching process, the pH value of the leaching solution is stabilized at 2.0, and the leaching reaction lasts for 60 min.

[0062] Using the same method for measuring the P dissolution rate as in Example 1, it was detected that after the modification treatment, the P dissolution rate in the ISSA of Example 2 was 95.71%.

[0063] ② Add an iron-containing substance to the leaching solution according to the Fe / P molar ratio of 1.1:1 and adjust the pH of the reaction system to 3.0; the iron-containing substance is a 0.5 mol / L Fe(NO3)3 solution, and the pH of the system is adjusted by adding an alkaline solution, and the alkaline solution is a Ca(OH)2 suspension with a concentration of 15 mol / L.

[0064] ③ Control the temperature within 40 °C to carry out a chemical precipitation reaction to form a FePO4 hydrate precipitate, and perform solid-liquid separation to obtain a FePO4 hydrate precipitate; the chemical precipitation reaction time is 2.0 h, and the stirring rate is controlled at 300 r / min.

[0065] ④ The hydrated precipitate obtained in step ③ was dried in an oven at 100 °C for 1.0 h and then calcined in a muffle furnace at 700 °C for 0.5 h to obtain a high-purity crystalline FePO4 product. The chemical composition of the product of Example 2 was analyzed by XRF, and the measurement results of the mass content of the chemical composition are listed in Table 3; the XRF analysis results (Table 3) show that the precipitated product prepared in Example 2 mainly consists of Fe2O3 (48.64%) and P2O5 (46.67%), while the impurity content is relatively low (<5%).

[0066] Example 3

[0067] A method for leaching and separating phosphorus from modified sludge incineration ash and preparing FePO4, the method comprising the following steps:

[0068] ① The sludge incineration ash ISSA was mixed with a modifier and subjected to high-temperature modification treatment, followed by leaching with dilute acid to separate a phosphorus-containing leachate and a residue; the modifier for the high-temperature modification treatment was CaO, and the addition amount of CaO was 40% of the mass of the sludge incineration ash; the temperature of the high-temperature modification treatment was 1100 °C. The dilute acid solution used in the acid leaching process was dilute HNO3, and the concentration of the dilute acid solution was 2.5 mol / L; the pH value of the leachate was stabilized at 3.5 during the acid leaching process, and the leaching reaction lasted for 80 min.

[0069] Using the same P dissolution rate measurement method as in Example 1, after detection, the P dissolution rate in the ISSA of Example 3 was significantly increased to 95.24% after the modification treatment.

[0070] ② Iron-containing substance was added to the leachate according to the Fe / P molar ratio of 1.2:1, and the pH of the reaction system was adjusted to 4.0; the iron-containing substance was a 0.5 mol / L Fe2(SO4)3 solution, and the pH of the system was adjusted by adding an alkaline solution, and the alkaline solution was ammonia water with a concentration of 5 mol / L.

[0071] ③ The chemical precipitation reaction was carried out at a temperature within 90 °C to form a FePO4 hydrate precipitate, and the solid-liquid separation was carried out to obtain a FePO4 hydrated precipitate; the chemical precipitation reaction time was 0.5 h, and the stirring rate was controlled at 30 r / min.

[0072] ④ The hydrated precipitate obtained in step ③ was dried in an oven at 110 °C for 0.5 h and then calcined in a muffle furnace at 300 °C for 2.0 h to obtain a high-purity crystalline FePO4 product. The chemical composition of the product of Example 3 was analyzed by XRF, and the measurement results of the mass content of the chemical composition are listed in Table 3; the XRF analysis results (Table 3) show that the precipitated product prepared in Example 3 mainly consists of Fe2O3 (45.42%) and P2O5 (48.26%), while the impurity content is relatively low (<7%).

[0073] Table 3 Chemical composition of the recovered FePO4 product in the examples (mass fraction / %)

[0074]

[0075] For any person skilled in the art, without departing from the scope of the technical solution of the present invention, many possible changes and modifications can be made to the technical solution of the present invention by using the technical content disclosed above, or it can be modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for leaching and separating phosphorus from reformed sludge incineration ash and preparing FePO4, characterized in that, The method includes the following steps: ① Mix the sludge incineration ash with a modifier and conduct high-temperature modification treatment, followed by leaching with dilute acid to separate the phosphorus-containing leachate and residue; ② Add an iron-containing substance to the leachate at an Fe / P molar ratio of 1 - 1.2:1, and adjust the pH of the reaction system to 1.0 - 4.0; ③ Control the temperature within the range of 40°C - 90°C to conduct a chemical precipitation reaction to form FePO4 hydrate precipitate; ④ The precipitate is dried and calcined to obtain the FePO4 product.

2. The method according to claim 1, wherein In step ①, the modifier for the high-temperature modification treatment is CaO, and the addition amount of CaO is 10% - 40% of the mass of the sludge incineration ash.

3. The method according to claim 1, characterized in that, The temperature of the high-temperature modification treatment in step ① is 800°C - 1100°C.

4. The method according to claim 1, wherein In the acid leaching process of step ①, the dilute acid solution used is dilute HCl, dilute HNO3 or dilute H2SO4 solution, and the concentration of the dilute acid solution is 0.5 - 2.5 mol / L; during the acid leaching process, the pH value of the leachate is stabilized at 1.0 - 3.5, and the leaching reaction lasts for 30 - 80 min.

5. The method according to claim 1, characterized in that, The iron-containing substance in step ② is selected from at least one of FeCl3, Fe(NO3)3 or Fe2(SO4)3.

6. The method according to claim 1, characterized in that, In step ②, the pH of the system is adjusted by adding an alkaline solution, and the alkaline solution is selected from at least one of NaOH solution, Ca(OH)2 suspension or ammonia water, and the concentration of the alkaline solution is 5 - 15 mol / L.

7. The method according to claim 1, wherein During the chemical precipitation reaction in step ③, the reaction time is 0.5 - 2.0 h, and the stirring rate is controlled at 30 - 300 r / min.

8. The method according to claim 1, wherein The drying conditions in step ④ are: drying temperature 60°C - 110°C, drying time 0.5 - 4.0 h.

9. The method according to claim 1, wherein The calcination treatment conditions in step ④ are: calcination temperature 300°C - 700°C, calcination time 0.5 - 2.0 h.

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