Method for recovering phosphorus and iron and use thereof

Abstract

The application discloses a method for recycling phosphorus and iron and application thereof, and comprises the following steps: S1: according to the amount of phosphorus-containing sewage, mixing an iron salt and the phosphorus-containing sewage to obtain a mixture, the concentration of the iron salt in the mixture is 4-25 mg / L, and the mixture is fermented at 20-60 DEG C for 2-10 days to obtain fermentation supernatant and remove impurities; S2: oxidizing the fermentation supernatant after the impurities are removed in an acid system. The application discloses a method for recycling phosphorus and iron, and realizes efficient recycling of phosphorus resources in sludge.

Description

Technical Field

[0001] This invention belongs to the field of wastewater resource utilization technology, specifically relating to a method for recovering phosphorus and iron and its application. Background Technology

[0002] Phosphate rock is a non-renewable non-metallic mineral resource on Earth, and phosphorus resources are expected to be depleted by the end of this century. In light of this situation, phosphorus recovery from phosphorus-rich municipal sewage sludge is an effective way to recycle and utilize phosphorus. The phosphorus content in dry municipal sewage sludge can be as high as 27.6 kg / ton. Effective recovery and efficient recycling of phosphorus resources from sludge can achieve effective resource utilization of the sludge.

[0003] The residual liquid after anaerobic fermentation of municipal sewage sludge is called anaerobic fermentation broth. During anaerobic fermentation, the complex composition of municipal sewage sludge releases nutrients and volatile organic compounds from the solid phase into the anaerobic fermentation broth. Therefore, anaerobic fermentation broth typically contains high concentrations of nitrogen, phosphorus, and organic matter, as well as abundant metallic elements such as iron, copper, zinc, manganese, and calcium. Current treatment methods for anaerobic fermentation broth primarily focus on removing pollutants to meet wastewater discharge standards. Simultaneously, due to its high nitrogen and phosphorus content, the broth can be recycled and reused as fertilizer, achieving resource recovery while addressing environmental pollution. The main recovery method involves using the anaerobic fermentation broth to prepare magnesium ammonium phosphate (struvite) or lapis lazuli as primary slow-release fertilizers for agricultural production; however, these methods also have several limitations. For struvite recycling, on the one hand, the theoretical preparation of struvite requires a molar ratio of phosphate, magnesium, and ammonium ions of 1:1:1. Due to the complex source and composition of sludge, the anaerobic fermentation broth obtained from anaerobic fermentation of sludge may have high ammonia nitrogen, low phosphorus, and insufficient magnesium ions. Therefore, a large amount of phosphate and magnesium ions need to be added, along with a large amount of alkaline solution to adjust the pH for struvite preparation, resulting in high production costs. On the other hand, due to the presence of a large number of metal elements in the fermentation broth, struvite is difficult to control in a targeted manner during preparation and recycling, posing a risk of heavy metal (Cu, Zn, etc.) pollution. This leads to low market acceptance as a fertilizer and risks environmental pollution. As for lavender, due to the difficulty in targeted control during recycling, the purity of lavender is low. After being generated during anaerobic digestion of sludge, it mixes with the sludge and is difficult to separate naturally. Furthermore, the recycled product is mainly amorphous and has high costs, limiting the efficient recycling of phosphorus from sludge.

[0004] Therefore, developing a method for recovering phosphorus and iron to achieve the economical and efficient recycling of phosphorus resources in sludge is currently a top priority. Summary of the Invention

[0005] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes a method for recovering phosphorus and iron, realizing the efficient recycling of phosphorus resources in sludge.

[0006] A method for recovering phosphorus and iron according to a first aspect of the present invention includes the following steps:

[0007] S1: Depending on the amount of phosphorus-containing wastewater, mix iron salt at 4-25 mg / L with the phosphorus-containing wastewater, ferment at 20-60℃ for 2-10 days, and then remove impurities from the fermentation supernatant.

[0008] S2: Oxidize the purified fermentation supernatant in an acidic system.

[0009] A method for recovering phosphorus and iron according to an embodiment of the present invention has at least the following beneficial effects:

[0010] This invention incorporates iron salts as a coagulant. After mixing with phosphorus-containing wastewater, the iron salts undergo physicochemical processes such as charge neutralization, bridging, and adsorption to destabilize the colloids, resulting in iron-based chemically enhanced primary sedimentation sludge (Fe-CEPS), classified as primary chemical sludge. Organic matter and phosphorus from the wastewater are enriched in the sludge. Fe-CEPS contains approximately 90% of the phosphorus from the wastewater, thus exhibiting high iron and phosphorus content. During fermentation, ferric iron is reduced to ferrous iron and released into the fermentation broth. Simultaneously, non-apatite inorganic phosphorus bound to the sludge is released as orthophosphate from the sludge solid phase into the fermentation supernatant. Targeting this portion of iron and phosphorus resources, without the addition of additional iron ions and phosphate, oxidation under acidic conditions causes co-precipitation of phosphorus and iron, achieving the recovery of phosphorus resources from the sludge in the form of ferric phosphate.

[0011] This invention is based on the anaerobic fermentation broth of Fe-CEPS and prepares iron phosphate crystals by co-precipitation. The preparation process does not require strict control of the iron and phosphorus concentrations during fermentation, which reduces the difficulty of iron phosphate preparation and makes it highly operable. At the same time, the co-precipitation method can basically achieve a 1:1 molar ratio of iron and phosphorus resources in the fermentation broth for recovery efficiency. The preparation method is simple and easy to implement, and has good recovery potential.

[0012] The above-mentioned amount of iron salt can ensure that the low binding efficiency of iron salt with phosphorus in wastewater is avoided due to insufficient iron salt, resulting in low phosphorus content in Fe-CEPS, which is difficult to use for the preparation of iron phosphate; at the same time, the excessive amount of iron salt and the tight binding state with phosphorus are avoided, resulting in too low phosphorus release and too high iron content during fermentation, which is also not conducive to the preparation of iron phosphate.

[0013] The fermentation temperature and time conditions described above can ensure the efficiency of iron and phosphorus leaching during fermentation.

[0014] According to some embodiments of the present invention, the iron salt includes at least one of ferrous sulfate (FeSO4), ferric chloride (FeCl3), ferric chloride hexahydrate (FeCl3·6H2O), and polyferric sulfate (PFS).

[0015] According to some embodiments of the present invention, step S1 further includes adding a polymeric coagulant.

[0016] According to some embodiments of the present invention, the polymeric coagulant includes at least one of activated silica, polyacrylamide, and sodium alginate.

[0017] According to some embodiments of the present invention, in step S2, the impurity removal step includes centrifuging and filtration of the fermentation supernatant.

[0018] According to some embodiments of the present invention, the centrifugation speed is 2000 to 8000 rpm.

[0019] According to some embodiments of the present invention, the centrifugation time is 3 to 10 minutes.

[0020] According to some embodiments of the present invention, the filter membrane in the membrane-passing step includes at least one of cellulose and polyethersulfone aqueous filter membranes.

[0021] According to some embodiments of the present invention, the pore size of the filter membrane is 0.3 to 0.45 μm.

[0022] According to some embodiments of the present invention, in step S2, the pH value of the acidic system is 1 to 4.

[0023] In this invention, by controlling the pH value of the acidic system to be between 1 and 4, the co-precipitation of other metal elements with phosphorus and iron due to a pH value higher than this is avoided, which would lead to an increase in other impurities. At the same time, the precipitation effect of iron phosphate would be reduced when the pH value is lower than this.

[0024] According to some embodiments of the present invention, the method for adjusting the acidic system is to add an acidic solution to the fermentation supernatant after impurity removal.

[0025] According to some embodiments of the present invention, the acidic solution includes at least one of sulfuric acid and hydrochloric acid.

[0026] According to some embodiments of the present invention, the molar concentration of the acidic solution is 1 to 6 mol / L.

[0027] According to some embodiments of the present invention, in step S2, the oxidizing agent includes hydrogen peroxide.

[0028] According to some embodiments of the present invention, the mass concentration of the hydrogen peroxide is 30% to 35%.

[0029] According to some embodiments of the present invention, the molar ratio of iron in the iron salt to hydrogen peroxide is 1:0.5 to 2.

[0030] According to some embodiments of the present invention, the oxidant is added by adding the oxidant while the system is being stirred.

[0031] According to some embodiments of the present invention, during the addition of the oxidant, the stirring speed is 150 to 350 rpm.

[0032] According to some embodiments of the present invention, the stirring time during the addition of the oxidant is 10 to 30 minutes.

[0033] During the process of adding the oxidant, a white precipitate appeared.

[0034] According to some embodiments of the present invention, in step S2, stirring continues after the white precipitate appears.

[0035] According to some embodiments of the present invention, in step S2, the stirring time is 30 to 300 minutes.

[0036] According to some embodiments of the present invention, the molar ratio of iron to phosphorus in the fermentation supernatant is 1:0.3 to 3.15.

[0037] In this invention, by controlling the reaction conditions, the molar ratio of iron to phosphorus in the fermentation broth is ensured to be 1:0.3 to 3.15. Then, by controlling the pH of the acidic system and adding hydrogen peroxide, the iron and phosphorus resources in the fermentation broth are precisely recovered in a targeted manner. This method can simply and effectively control most other cationic impurities and organic matter to remain in the solution, greatly expanding the source of raw materials for iron phosphate. The obtained iron phosphate is spherical or near-spherical nano-sized iron phosphate crystals with few impurities and high purity.

[0038] According to some embodiments of the present invention, the temperature of the acidic system is 20–90°C.

[0039] According to some embodiments of the present invention, the fermentation method includes anaerobic fermentation and combined fermentation.

[0040] According to some embodiments of the present invention, the co-fermentation is carried out by adding organic matter to a mixture of the iron salt and the wastewater.

[0041] Under the same conditions, co-fermentation has a better iron and phosphorus release effect than anaerobic fermentation because exogenous organic matter provides a sufficient carbon source, which can promote the release of iron and phosphorus while anaerobic acidification.

[0042] According to some embodiments of the present invention, the oxidation process further includes impurity removal.

[0043] According to some embodiments of the present invention, the method for removing impurities includes calcination.

[0044] In this invention, the fermentation broth after fermentation contains a large amount of organic matter. The release of iron and phosphorus is inevitably accompanied by the release and dissolution of organic matter. While iron phosphate precipitates, some organic matter will also co-precipitate. Therefore, this part of the organic matter can be removed by calcination.

[0045] According to some embodiments of the present invention, the calcination includes a first heat treatment and a second heat treatment.

[0046] According to some embodiments of the present invention, the temperature of the first heat treatment is 350–450°C.

[0047] According to some embodiments of the present invention, the temperature of the second heat treatment is 650–900°C.

[0048] According to some embodiments of the present invention, the carrier gas atmosphere for sintering is air or high-purity oxygen.

[0049] According to some embodiments of the present invention, after the continued stirring, before the impurity removal and calcination, the process also includes heat preservation and aging, solid-liquid separation, washing, drying, and grinding.

[0050] According to some embodiments of the present invention, the heat preservation and aging time is 10 to 24 hours.

[0051] According to some embodiments of the present invention, the solid-liquid separation method includes at least one of centrifugation through a membrane and high-speed centrifugation.

[0052] According to some embodiments of the present invention, the washing solvent includes at least one selected from deionized water, high-purity water, anhydrous ethanol, and acetone.

[0053] According to some embodiments of the present invention, the number of washing cycles is 3 to 6.

[0054] According to some embodiments of the present invention, the drying process includes primary drying and secondary drying.

[0055] According to some embodiments of the present invention, the drying time for one cycle is 5 to 24 hours.

[0056] According to some embodiments of the present invention, the temperature of the primary drying is 30-50°C.

[0057] According to some embodiments of the present invention, the drying time for one drying cycle is 10 to 24 hours.

[0058] According to some embodiments of the present invention, the temperature of the primary drying is 80–150°C.

[0059] According to some embodiments of the present invention, the grinding includes at least one of manual grinding and ball milling.

[0060] According to some embodiments of the present invention, the rotational speed of the ball mill is 300 to 500 rpm.

[0061] According to some embodiments of the present invention, the ball milling time is 10 to 24 hours.

[0062] According to some preferred embodiments of the present invention, the method for recovering phosphorus and iron includes the following steps:

[0063] A1: Take the wastewater after grid treatment, add iron salt for coagulation and sedimentation to prepare Fe-CEPS, put it into a fermentation bottle for constant temperature anaerobic fermentation to achieve iron and phosphorus dissolution;

[0064] A2: Centrifuge the fermentation broth from step A1 at high speed and pass it through a membrane quickly to remove solid impurities;

[0065] A3: Place the fermentation supernatant after centrifugation and membrane separation in step A2 into a beaker container, place it on a constant temperature magnetic stirrer and stir at a constant temperature and speed while adding acid solution to adjust the pH value to 1-4.

[0066] A4: Add hydrogen peroxide to the fermentation broth from step A3 for oxidation and continue stirring until a white precipitate appears. Continue stirring.

[0067] A5: Stop stirring and continue to keep warm and age. Then, through solid-liquid separation, washing and drying, grinding and sintering, the finished iron phosphate crystal product is obtained.

[0068] Application of the above-described method according to a second aspect of the present invention in the preparation of lithium iron phosphate. Attached Figure Description

[0069] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0070] Figure 1 This is a flowchart of the method in the embodiment;

[0071] Figure 2 The ICP elemental diagram of the iron phosphate prepared in Example 1 is shown below.

[0072] Figure 3 The XRD pattern of the iron phosphate prepared in Example 1 is shown below.

[0073] Figure 4 Here is a SEM image of the iron phosphate prepared in Example 1;

[0074] Figure 5 Here is a SEM image of the iron phosphate prepared in Example 1;

[0075] Figure 6 Here is a SEM image of the iron phosphate prepared in Example 1;

[0076] Figure 7 The image shows a SEM image of the iron phosphate prepared in Example 1. Detailed Implementation

[0077] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0078] Example 1

[0079] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate, such as... Figure 1 As shown, the specific steps include:

[0080] A1: Take 450 mL of chemically enhanced primary sludge (SS 8.33 g / L, VSS 5.20 g / L) prepared by adding polyferric sulfate (PFS) to raw water from the sewage treatment plant at an iron content of 10 mg / L and put it into a serum bottle. Aerate with nitrogen (99.99%) for 5 min and carry out anaerobic fermentation for 6 days at 37℃ and 150 rpm in a constant temperature shaker.

[0081] A2: Take out the fermentation supernatant obtained from anaerobic fermentation in step A1, centrifuge it at 4000 rpm for 5 min in a high-speed centrifuge, and filter the supernatant through a 0.45 μm filter membrane to remove solid impurities;

[0082] A3: Place the supernatant after membrane filtration in step A2 on a constant temperature magnetic stirrer and stir at a constant speed of 250 rpm under a constant temperature of 40℃. Slowly add 1:1 H2SO4 solution to the fermentation broth and adjust the pH of the solution to 1.8.

[0083] A4: Slowly add 30% hydrogen peroxide dropwise to the fermentation broth obtained in step A3 to oxidize the ferrous ions in the anaerobic fermentation supernatant into ferric ions. The amount added is 1:1 molar ratio of ferric ions to hydrogen peroxide. A white precipitate appears. Continue stirring for 2 hours.

[0084] A5: After stirring for 2 hours in step A4, stop stirring and continue to keep warm for 12 hours. Separate the white precipitate and supernatant by passing them through a membrane. Wash the precipitate three times with ultrapure water, dry it, grind it manually, and sinter it in air at 700℃ for 12 hours to obtain the product iron phosphate.

[0085] Figure 1This is a flowchart of the method in the embodiment;

[0086] Figure 2 The image shows the ICP elemental diagram of the iron phosphate prepared in Example 1. The ICP results show that iron and phosphorus are the main constituent elements in the iron phosphate recovered through acidification fermentation and oxidation, with a molar ratio of approximately 1:1. Calcium (Ca), potassium (K), and sodium (Na) are the main metallic impurities, with a total relative mass fraction of approximately 0.36%. Other metallic impurities were not detected.

[0087] Figure 3 The XRD pattern of the iron phosphate prepared in Example 1 shows that the prepared iron phosphate has sharp and prominent peaks that highly overlap with the iron phosphate standard card (PDF#84-0876), and there are no obvious impurity peaks.

[0088] Figures 4-7 The image shows a SEM image of the iron phosphate prepared in Example 1. The results show that the prepared iron phosphate is a nanosphere iron phosphate with a regular structure and a diameter between 100 and 300 nm.

[0089] Example 2

[0090] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate, such as... Figure 1 As shown, the specific steps include:

[0091] A1: Take 450 mL of chemically enhanced primary sludge (SS 9.62 g / L, VSS 6.35 g / L) prepared by adding ferric chloride (FeCl3) to raw water from the sewage treatment plant and add it to a serum bottle. Aerate with nitrogen (99.99%) for 3 min and carry out anaerobic fermentation for 5 days at 37℃ and 150 rpm in a constant temperature shaker.

[0092] A2: Take out the fermentation supernatant obtained from anaerobic fermentation in step A1, centrifuge it at 4500 rpm for 5 min in a high-speed centrifuge, and filter the supernatant through a 0.45 μm filter membrane to remove solid impurities;

[0093] A3: Place the supernatant after membrane filtration in step A2 on a constant temperature magnetic stirrer and stir at a constant speed of 250 rpm at 50℃. Slowly add concentrated sulfuric acid to the fermentation broth to adjust the pH of the solution to 1.8.

[0094] A4: Slowly add 30% hydrogen peroxide dropwise to the fermentation broth obtained in step A3 to oxidize the ferrous ions in the anaerobic fermentation supernatant to ferric ions. The amount added is 1:2 molar ratio of ferric ions to hydrogen peroxide. A white precipitate appears. Continue stirring for 1.5 hours.

[0095] A5: After stirring for 1.5 hours in step A4, stop stirring and continue to keep warm for 10 hours. Separate the white precipitate and supernatant by passing them through a membrane. Wash once with ultrapure water and twice with ethanol, then dry. Grind manually and sinter at 650°C in air for 15 hours to obtain the product ferric phosphate.

[0096] Example 3

[0097] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate, such as... Figure 1 As shown, the specific steps include:

[0098] A1: Take 450 mL of chemically enhanced primary sludge (SS 10.21 g / L, VSS 5.46 g / L) prepared by adding ferric sulfate (FeSO4) to the raw water from the sewage treatment plant and a small amount of polyacrylamide to a conical flask, aerate with nitrogen (99.99%) for 10 min, and carry out anaerobic fermentation for 8 days at 37℃ and 200 rpm in a constant temperature shaker.

[0099] A2: Take out the fermentation supernatant obtained from anaerobic fermentation in step A1, centrifuge at 5000 rpm for 4 min in a high-speed centrifuge, and filter the supernatant through a 0.45 μm filter membrane to remove solid impurities;

[0100] A3: Place the supernatant after membrane filtration in step A2 on a constant temperature magnetic stirrer and stir at 280 rpm at 30℃. Slowly add 6 mol / L H2SO4 solution to the fermentation broth and adjust the pH of the solution to 2.2.

[0101] A4: Slowly add 30% hydrogen peroxide dropwise to the fermentation broth obtained in step A3 to oxidize the ferrous ions in the anaerobic fermentation supernatant into ferric ions. The amount added is 1:0.5 molar ratio of ferric ions to hydrogen peroxide. A white precipitate appears. Continue stirring for 4 hours.

[0102] A5: After stirring for 4 hours in step A4, stop stirring and continue to keep warm for 10 hours. Separate the white precipitate and supernatant by passing them through a membrane. Wash twice with ultrapure water and twice with ethanol, then dry. Grind manually and sinter at 800℃ in an oxygen atmosphere for 16 hours to obtain the product ferric phosphate.

[0103] Example 4

[0104] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate, such as... Figure 1 As shown, the specific steps include:

[0105] A1: Take 450 mL of chemically enhanced primary sludge (SS 4.48 g / L, VSS 2.17 g / L) prepared by adding ferric chloride hexahydrate (FeCl3·6H2O) to raw water from the sewage treatment plant and add it to a serum bottle. Aerate with nitrogen (99.99%) for 5 min and carry out anaerobic fermentation for 5 days in a constant temperature shaker at 37℃ and 200 rpm.

[0106] A2: Take out the fermentation supernatant obtained from anaerobic fermentation in step A1, centrifuge it at 4000 rpm for 6 min in a high-speed centrifuge, and filter the supernatant through a 0.45 μm filter membrane to remove solid impurities;

[0107] A3: Place the supernatant after membrane filtration in step A2 on a constant temperature magnetic stirrer and stir at 300 rpm at 80℃. Slowly add 1:1 HCl solution to the fermentation broth and adjust the pH of the solution to 1.7.

[0108] A4: Slowly add 30% hydrogen peroxide dropwise to the fermentation broth obtained in step A3 to oxidize the ferrous ions in the anaerobic fermentation supernatant into ferric ions. The amount added is 1:1.5 molar ratio of ferric ions to hydrogen peroxide. A white precipitate appears. Continue stirring for 3 hours.

[0109] A5: After stirring for 3 hours in step A4, stop stirring and continue to keep warm for 10 hours. Separate the white precipitate and supernatant by passing them through a membrane. Wash with deionized water three times and dry. Grind manually and sinter at 800℃ in an oxygen atmosphere for 10 hours to obtain the product ferric phosphate.

[0110] Comparative Example 1

[0111] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare ferric phosphate. The difference between this comparative example and Example 1 is that ferric sulfate is mixed with phosphorus-containing wastewater at a concentration of 3 mg / L, while the other conditions are the same.

[0112] In this comparative example, the amount of ferric sulfate was too small, resulting in low binding efficiency with phosphorus in the wastewater. The phosphorus content in Fe-CEPS was also low, making it difficult to use for the preparation of ferric phosphate.

[0113] Comparative Example 2

[0114] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare ferric phosphate. The difference between this comparative example and Example 1 is that ferric sulfate is mixed with phosphorus-containing wastewater at a concentration of 28 mg / L, while the other conditions are the same.

[0115] In this comparative example, the excess of ferric sulfate resulted in a tighter binding with phosphorus, leading to a low phosphorus release and an excessively high iron content during fermentation, which is detrimental to the preparation of ferric phosphate.

[0116] Comparative Example 3

[0117] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate. The difference between this comparative example and Example 1 is that the fermentation temperature is 30°C, while the other conditions are the same.

[0118] The fermentation temperature in this comparative example was too low, resulting in a low release of iron and phosphorus from Fe-CEPS, making it difficult to effectively recover phosphorus from wastewater and added iron.

[0119] Comparative Example 4

[0120] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate. The difference between this comparative example and Example 1 is that the fermentation temperature is 70°C, while the other conditions are the same.

[0121] In this comparative example, the fermentation temperature was too high, causing iron-reducing bacteria in the sludge to lose their activity, making it difficult to maintain the acidification fermentation process, resulting in poor iron and phosphorus release and making it difficult to recycle.

[0122] Comparative Example 5

[0123] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate. The difference between this comparative example and Example 1 is that the pH value in step A3 is 0.5, while the other conditions are the same.

[0124] In this comparative example, the pH value was too low. Under strongly acidic conditions, the iron and phosphorus dissolved and released into the solution could not be effectively co-precipitated, making it difficult to prepare iron phosphate.

[0125] Comparative Example 6

[0126] This embodiment provides a method for separating and recovering iron and phosphorus resources from Fe-CEPS to prepare iron phosphate. The difference between this comparative example and Example 1 is that the pH value in step A3 is 5, while the other conditions are the same.

[0127] In this comparative example, the pH is already close to neutral. If iron phosphate is precipitated at this time, impurities such as magnesium phosphate will be introduced for co-precipitation, resulting in an increase in the content of impurities in the recovered iron phosphate.

[0128] The above description of specific exemplary embodiments of the present invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed. Due to my country's vast territory and regional differences, the composition and properties of sewage sludge from wastewater treatment plants vary, resulting in relatively complex compositions. Furthermore, it is obvious to those skilled in the art that various corresponding changes and modifications can be made based on the above technical solutions and concepts. The purpose of selecting and describing exemplary embodiments is to reveal the specific principles and practical applications of the present invention, thereby enabling those skilled in the art to implement and utilize various different exemplary experimental schemes and various selections and modifications of the present invention. All such changes and modifications are defined by the claims and their equivalents.

Claims

1. A method for recovering phosphorus and iron, characterized in that, Includes the following steps: S1: Based on the amount of phosphorus-containing wastewater, iron salts and phosphorus-containing wastewater are mixed to obtain a mixture. The concentration of iron salts in the mixture is 4~25 mg / L. The mixture is fermented at 20~60℃ for 2~10 days. The supernatant of the fermentation is then removed to remove impurities. S2: The fermentation supernatant after impurity removal is oxidized by adding hydrogen peroxide in an acidic system with a pH of 1 to 4, wherein the molar ratio of iron in the iron salt to hydrogen peroxide is 1:0.5 to 2. The oxidation process is followed by impurity removal, and the impurity removal method is calcination. The method described herein enables the recovery of phosphorus resources from sludge in the form of iron phosphate, wherein the iron phosphate is spherical or near-spherical nanoscale iron phosphate crystals.

2. The method according to claim 1, characterized in that, In the fermentation supernatant, the molar ratio of iron to phosphorus is 1:0.3~3.

15.

3. The method according to claim 1, characterized in that, The temperature of the acidic system is 20~90℃.

4. The method according to claim 1, characterized in that, The fermentation methods include anaerobic fermentation and combined fermentation.

5. The method according to claim 4, characterized in that, The co-fermentation involves adding organic matter to a mixture of the iron salt and the wastewater to carry out the co-fermentation.

6. The method according to claim 1, characterized in that, The calcination includes a first heat treatment and a second heat treatment.

7. The method according to claim 6, characterized in that, The temperature of the first heat treatment is 350~450℃.

8. The method according to claim 6, characterized in that, The temperature of the second heat treatment is 650~900℃.

9. The application of the method as described in any one of claims 1 to 8 in the preparation of lithium iron phosphate.

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