A method for recovering and utilizing nitrogen and phosphorus resources from wastewater

Abstract

The present application relates to a kind of methods for recycling nitrogen phosphorus resources from wastewater and utilization, comprising the following steps: S1: wastewater is concentrated, to improve the concentration of ammonia nitrogen in wastewater, facilitate subsequent reaction;S2: concentrated liquid containing ammonia nitrogen is added to reaction kettle, then add phosphorus salt and ferrous salt, carry out crystallization reaction, generate complex salt ferrous ammonium phosphate;S3: ferrous ammonium phosphate is added into pyrolysis furnace, and pyrolysis is carried out under oxygen atmosphere, obtain iron phosphate;S4: ammonia gas discharged from pyrolysis furnace is collected, then absorbed with acid liquor, and made into liquid nitrogen fertilizer.The present application uses phosphorus salt and ferrous salt to react with wastewater, simultaneously removes nitrogen and phosphorus, generates difficultly soluble complex salt ferrous ammonium phosphate, then directional pyrolysis in oxidizing environment, converts ferrous ammonium phosphate into iron phosphate, thereby recycling phosphorus;Ammonia gas produced by pyrolysis is absorbed with acid liquor to recycle nitrogen, with high absorption efficiency, thereby realizing simultaneous removal of nitrogen and phosphorus and high-value recycling.

Description

Technical Field

[0001] This invention belongs to the field of wastewater resource utilization technology, specifically relating to a method for recovering and utilizing nitrogen and phosphorus resources from wastewater. Background Technology

[0002] Nitrogen and phosphorus are the main culprits of eutrophication in water bodies. Excessive discharge of nitrogen and phosphorus into water bodies causes water quality deterioration and imbalance of aquatic ecosystems. On the other hand, nitrogen and phosphorus are important industrial and agricultural raw materials, used in huge quantities. Recovering nitrogen and phosphorus from wastewater has both economic and environmental benefits. Currently, the main methods for simultaneous nitrogen and phosphorus recovery from wastewater are adsorption and crystallization. Both methods use struvite as a reaction medium and can simultaneously remove and recover nitrogen and phosphorus from wastewater with low and high concentrations. However, at present, the cost of reagents for struvite technology is high, while the value of the resulting struvite product is low, only suitable as a fertilizer additive or industrial raw material. As a result, nitrogen and phosphorus recovery technologies with struvite as the final product are often difficult to promote due to poor economics. Those skilled in the art need to consider targeted value-added treatment of nitrogen and phosphorus products. Summary of the Invention

[0003] To address the above problems, this invention provides a method for recovering and utilizing nitrogen and phosphorus resources from wastewater, comprising the following steps:

[0004] S1: Concentrate the wastewater to increase the concentration of ammonia nitrogen in the wastewater, which will facilitate subsequent reactions;

[0005] S2: Add the concentrated solution containing ammonia nitrogen to the reaction vessel, then add phosphate salt and ferrous salt to carry out the crystallization reaction and generate the double salt ferrous ammonium phosphate.

[0006] S3: Ferrous ammonium phosphate is added to a pyrolysis furnace and pyrolyzed in an oxygen atmosphere to obtain ferric phosphate;

[0007] S4: Collect the ammonia gas discharged from the pyrolysis furnace, then absorb it with acid to produce liquid nitrogen fertilizer.

[0008] This invention uses phosphate and ferrous salts to react with wastewater to simultaneously remove nitrogen and phosphorus, generating a sparingly soluble double salt, ferrous ammonium phosphate. This ferrous ammonium phosphate is then directionally pyrolyzed in an oxidizing environment to convert it into ferric phosphate, thereby recovering phosphorus. The ammonia gas generated during pyrolysis is absorbed by acid to recover nitrogen, resulting in high absorption efficiency. This achieves simultaneous removal and high-value recovery of nitrogen and phosphorus.

[0009] Optionally, in step S1, a two-stage electrodialysis method is used to concentrate the wastewater, specifically including the following steps:

[0010] (1) Raw material wastewater is fed into the first-stage electrodialysis unit for electrodialysis. The concentrated wastewater generated in the concentrated water chamber of the first-stage electrodialysis unit is collected, and the fresh water generated in the fresh water chamber is used as recycled water.

[0011] (2) The concentrated wastewater generated in step (1) is fed into a secondary electrodialysis device for electrodialysis to reduce the hardness of the concentrated wastewater and obtain the concentrated liquid containing ammonia nitrogen.

[0012] The primary electrodialysis device is a conventional electrodialysis device, in which several ion exchange membranes are set between the cathode plate and the anode plate, and cation exchange membranes and anion exchange membranes are set alternately to form alternating concentrate chambers and desalination chambers.

[0013] The secondary electrodialysis device is equipped with a cathode plate, a cation exchange membrane, and an anode plate in sequence. Divalent cations (calcium ions and magnesium ions) in the concentrated wastewater enter the chamber between the cation exchange membrane and the cathode plate, while the concentrated wastewater with reduced hardness remains in the chamber between the cation exchange membrane and the anode plate. The concentration of ammonia nitrogen in the concentrated solution containing ammonia nitrogen is not less than 5.6 g / L.

[0014] Optionally, in step S2, the phosphate salt is selected from hydrogen phosphate or dihydrogen phosphate, and the ferrous salt is selected from ferrous chloride, ferrous sulfate, or ferrous nitrate.

[0015] Optionally, in step S2, the molar ratio of iron, phosphorus, and nitrogen in the reactor is (1-2):(1-2):1. The pH of the reaction solution is controlled between 8.0 and 10.0 using NaOH or KOH. The reaction is carried out at 60-80℃ for 0.5-2.0 h, and the stirring rate is controlled at 60-120 rpm.

[0016] Optionally, in step S2, the concentration of humic acid in the concentrated solution containing ammonia nitrogen is 300-500 mg / L, and Cl... - The concentration is 700-1000 mg / L, CO3 2- The concentration is 50-70 mg / L.

[0017] The inventors unexpectedly discovered that the concentrated solution containing ammonia nitrogen obtained after two-stage electrodialysis contained humic acid and Cl. - CO3 2- These ions can improve the reaction efficiency and lower the reaction temperature in step S2, while humic acid can be carbonized during the pyrolysis in step S3, thus being eliminated as CO2 and water, without affecting the quality of ferric phosphate. The concentrated solution containing ammonia nitrogen contains humic acid and Cl... - CO3 2- If the concentration does not reach the above range, additional substances can be added to increase the concentration of humic acid and Cl. - CO3 2- The concentration reaches the above range. Calcium and magnesium ions in the concentrated solution containing ammonia nitrogen have an adverse effect on the reaction in step S2 and can be removed by step (2) above.

[0018] Optionally, in step S3, the product from step S2 is filtered, fed into a pyrolysis furnace, oxygen is introduced, the temperature is raised at a rate of 8-10℃ / min, the temperature is raised to 800-900℃, maintained for 30-50min, and then cooled to obtain the final product, iron phosphate.

[0019] Optionally, in step S4, the acid solution is selected from sulfuric acid solution, hydrochloric acid solution or nitric acid solution, and the concentration of the acid solution is adaptively adjusted according to the amount of ammonia gas discharged from the pyrolysis furnace and the required concentration of liquid nitrogen fertilizer.

[0020] Existing pyrolysis furnaces are mainly tube furnaces and high-temperature rotary furnaces. Tube furnaces generally have thinner tubes, suitable for small-scale experimental studies, but the samples are uniformly stacked in a sample boat, resulting in uneven heating. High-temperature rotary furnaces have a larger, rotatable furnace chamber, mainly used in gaseous deposition. Solid samples rotate along the inner wall of the furnace chamber, which is unsuitable for the pyrolysis reaction of this invention because the rotation of the solid samples along the furnace wall hinders sufficient contact with oxygen, leading to incomplete reaction. Therefore, this invention provides a novel tube furnace with a rotating sample holder inside, which facilitates sufficient contact with oxygen.

[0021] Optionally, the novel tubular furnace includes a furnace tube and a sample rack inside the furnace tube. The furnace tube is a hollow cylinder and is horizontal. An oxygen inlet is provided at the lower part of one end of the furnace tube, and a gas outlet is provided at the other end for discharging the generated ammonia and excess oxygen.

[0022] One end of the sample holder extends out of the furnace tube and is connected to a drive motor outside the furnace tube to drive the sample holder to rotate. The sample holder includes several sample trays and several support rods. The sample trays are evenly distributed along the axial direction of the furnace tube and are concentric with the furnace tube. The support rods are parallel to the axial direction of the furnace tube and are evenly distributed along the circumference of the sample trays. The support rods surround the edge of the sample trays and are connected to the edge of the sample trays, so that the sample trays are connected in a series.

[0023] Further optionally, the sample tray is a flat cylinder with its outer walls made of mesh, which allows oxygen to enter and exit the sample tray while preventing solid materials inside the sample tray from leaking out.

[0024] The circular surface of the sample tray is perpendicular to the axis of the furnace tube. The sample tray is equipped with several intersecting partitions, which divide the internal space of the sample tray into several grid-like sub-spaces, so as to facilitate the uniform distribution of the sample in the sample tray.

[0025] The sample pan has a uniform corrugated mesh facing the oxygen inlet, which helps to agitate the oxygen on the upstream side of the sample pan and promote gas-solid contact.

[0026] Each sample tray has a gap between itself and the inner wall of the furnace tube, serving as a gas passage. This allows oxygen to not only cross the surface of the sample tray (to react with the solid sample inside), but also flow through the gas passage to the next sample tray. This prevents excessive resistance when oxygen crosses the sample trays, which could lead to high pressure inside the furnace tube. Furthermore, the gas passages for adjacent sample trays are not aligned; instead, they are arranged in a clockwise or counterclockwise rotational pattern. This creates a rotating flow pattern within the furnace tube, promoting oxygen agitation and facilitating full contact between the oxygen and the solid sample. To ensure smooth rotation of the sample holder within the furnace tube, very small gaps are maintained between the sample trays and the inner wall of the furnace tube in other directions. These gaps are much smaller than the gas passages and do not significantly impede the rotation of the sample holder.

[0027] Before pyrolysis, solid ferrous ammonium phosphate is uniformly filled into the compartments of each sample tray and then placed into the furnace tube. One end of each support rod converges and extends out of the furnace tube before connecting to the drive motor. A rotating seal is installed at the connection point between the support rod and the furnace tube to prevent air leakage. The drive motor rotates the entire sample rack while the furnace tube remains stationary. A heating device is installed outside the furnace tube to provide heat for the pyrolysis reaction. The dimensions of the furnace tube and sample trays, as well as the number of sample trays, are rationally set according to the amount of ferrous ammonium phosphate to be processed. Oxygen is introduced into the furnace tube, and the sample rack rotates. The arrangement of the sample trays and their compartments greatly prevents sample accumulation. Oxygen passing over the surface of the sample trays facilitates oxygen dispersion, thereby promoting sufficient contact between oxygen and the sample and increasing the yield of ferric phosphate.

[0028] In practice, uniformly dispensing the double salt ferrous ammonium phosphate into the compartments of each sample tray is a tedious task. Manual operation suffers from cumbersome weighing and uneven sample distribution. Mechanical operation is costly and time-consuming, and the solids can easily contaminate the operating environment. This application proposes the following solution.

[0029] Optionally, in step S2, a clean sample holder is placed inside the reaction vessel and used as a stirrer. One end of the sample holder extends upward through the reaction vessel and is connected to a stirring motor to drive the sample tray to rotate.

[0030] The sample tray is placed horizontally with the corrugated side facing upwards.

[0031] Step S2 is a crystallization reaction. The stirrer facilitates the reaction and controls crystal growth. The sample holder, placed inside the reaction vessel, not only provides excellent stirring but also acts as a crystal carrier. At the start of the reaction, the concentrated solution containing ammonia nitrogen, phosphate salts, and ferrous salts fully contact and react under stirring. The resulting small crystal particles are not intercepted by the mesh of the sample tray and can freely enter and exit the tray. When the crystals grow to a certain size, once inside the tray, they easily adhere to the mesh or partition under hydraulic pressure. After attachment, the crystals act as nuclei and continue to grow. When the crystal size exceeds the mesh aperture of the sample tray, it remains inside and can continue to grow. Due to the cage-like structure of the sample trays and the arrangement of several trays, most crystals and nuclei can be captured in the rotating water. The uniform hydraulic environment during stirring facilitates the even entry of crystals and nuclei into the various compartments of each sample tray.

[0032] Because crystals have their own weight, there will be more crystals in the lower part of the reactor. To prevent excessive crystals in the sample tray at the bottom of the reactor, the reactor can optionally be equipped with an aeration device at the bottom to provide upward disturbance to the middle and lower parts of the reactor. The bubbles adhere to the surface of the crystals, which helps to carry the crystals back to the middle and upper parts of the reactor, thus balancing the crystals in the upper and lower parts of the reactor. Even if a small number of bubbles carry the crystals to the surface, the bubbles will burst when they encounter atmospheric pressure, and the crystals can fall back down and return to the middle and upper parts of the reactor to continue the reaction. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the tube furnace in Example 12;

[0034] Figure 2 This is a schematic diagram of the sample tray;

[0035] Figure 3 This is a schematic diagram of the reactor structure in Example 12.

[0036] In the attached diagram, 1-furnace tube, 2-sample rack, 3-oxygen inlet, 4-gas outlet, 5-sample tray, 6-support rod, 7-partition plate, 8-dividing space, and 9-reaction vessel. Detailed Implementation

[0037] The water quality of the raw material wastewater used in the following examples and comparative examples is as follows: PO4 3- The concentration is 230 mg / L, NH 4+ The concentration was 1800 mg / L, the humic acid concentration was 54 mg / L, and the Cl concentration was... - The concentration is 265 mg / L, CO3 2- The concentration is 13 mg / L.

[0038] Example 1

[0039] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater as described in this embodiment includes the following steps:

[0040] S1: Concentrate the wastewater to increase the concentration of ammonia nitrogen in the wastewater, which will facilitate subsequent reactions;

[0041] S2: Add the concentrated solution containing ammonia nitrogen to the reaction vessel, then add KH2PO4 and FeCl2 to carry out the crystallization reaction and generate the double salt ferrous ammonium phosphate;

[0042] S3: Ferrous ammonium phosphate is added to a pyrolysis furnace and pyrolyzed in an oxygen atmosphere to obtain ferric phosphate;

[0043] S4: Collect the ammonia gas discharged from the pyrolysis furnace, then absorb it with acid to produce liquid nitrogen fertilizer.

[0044] In step S1, the wastewater is concentrated using a two-stage electrodialysis method, specifically including the following steps:

[0045] (1) Raw material wastewater is fed into the first-stage electrodialysis unit for electrodialysis. The concentrated wastewater generated in the concentrated water chamber of the first-stage electrodialysis unit is collected, and the fresh water generated in the fresh water chamber is used as recycled water.

[0046] (2) The concentrated wastewater generated in step (1) is fed into a secondary electrodialysis device for electrodialysis to reduce the hardness of the concentrated wastewater and obtain the concentrated liquid containing ammonia nitrogen.

[0047] The primary electrodialysis device is a conventional electrodialysis device, in which several ion exchange membranes are installed between the cathode plate and the anode plate, with cation exchange membranes and anion exchange membranes alternating to form alternating concentrate and desalination chambers. The ammonia nitrogen concentration of the produced desalination water is below 70 mg / L, and the phosphate concentration is below 8 mg / L.

[0048] The secondary electrodialysis device is equipped with a cathode plate, a cation exchange membrane, and an anode plate arranged sequentially. Cations (calcium and magnesium ions) in the concentrated wastewater enter the chamber between the cation exchange membrane and the cathode plate, while the hardness-reduced concentrated wastewater remains in the chamber between the cation exchange membrane and the anode plate. The ammonia nitrogen concentration of the concentrated solution containing ammonia nitrogen is 5.8 g / L. The cation exchange membrane used is an Astom cation exchange membrane from Japan, which can effectively separate divalent cations.

[0049] Before step (1), the wastewater undergoes traditional pretreatment of coagulation, sedimentation, and filtration.

[0050] In step S2, the concentrated solution containing ammonia nitrogen in the reactor contains humic acid at a concentration of 300 mg / L and Cl... - The concentration was 700 mg / L, CO3 2- The concentration was 50 mg / L;

[0051] Meanwhile, in the reactor, the molar ratio of iron, phosphorus, and nitrogen is 2:2:1, and the pH of the reaction solution is controlled at 10 using NaOH. The reaction is carried out at 80℃ for 0.5 hours with a stirring rate of 60 rpm.

[0052] In step S3, the product from step S2 is filtered and then fed into a pyrolysis furnace. Oxygen is introduced, and the temperature is raised to 800°C at a rate of 10°C / min, maintained for 30 minutes, and then cooled to obtain the final product, ferric phosphate. The gas discharged from the pyrolysis furnace is fed into a sulfuric acid absorption tank to produce liquid nitrogen fertilizer. The sulfuric acid absorption tank contains a 30-50 wt% sulfuric acid solution. The pyrolysis furnace is a conventional tubular furnace.

[0053] Comparative Example 1

[0054] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this comparative example is the same as that in Example 1, except that the concentration step S1 is omitted, and the raw material wastewater is directly added to the reactor for step S2.

[0055] Example 2

[0056] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1. The difference is that in step S1, sulfonic acid-based ammonia nitrogen removal resin is used to adsorb ammonia nitrogen in the wastewater, and then the resin is eluted with an eluent (5wt% hydrochloric acid solution) to obtain an eluent. After elution, the resin is regenerated and rinsed with 5wt% sodium hydroxide solution. The regenerated resin can be used to concentrate wastewater again.

[0057] The eluent is a concentrated solution containing ammonia nitrogen, with an ammonia nitrogen concentration of 5.8 g / L, humic acid, and Cl-. - CO3 2- The concentration is essentially zero.

[0058] Example 3

[0059] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the secondary electrodialysis in step (2) is not performed, i.e., calcium and magnesium ions are not removed, and the concentration of calcium and magnesium ions in the concentrate containing ammonia nitrogen is 100 mg / L.

[0060] Example 4

[0061] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the concentration of humic acid in the concentrated solution containing ammonia nitrogen obtained in step S1 is 500 mg / L.

[0062] Example 5

[0063] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the concentration of humic acid in the concentrated solution containing ammonia nitrogen obtained in step S1 is 550 mg / L.

[0064] Example 6

[0065] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the concentrated solution containing ammonia nitrogen obtained in step S1 contains Cl - The concentration is 1000 mg / L.

[0066] Example 7

[0067] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the concentrated solution containing ammonia nitrogen obtained in step S1 contains Cl - The concentration was 1100 mg / L.

[0068] Example 8

[0069] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the concentrated solution containing ammonia nitrogen obtained in step S1 contains CO3. 2- The concentration was 70 mg / L.

[0070] Example 9

[0071] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the concentrated solution containing ammonia nitrogen obtained in step S1 contains CO3. 2- The concentration was 80 mg / L.

[0072] Table 1 Comparison of the effects of Examples 1-9

[0073] N / P removal molar ratio N / P removal molar ratio Example 1 0.93 Example 6 0.95 Example 2 0.81 Example 7 0.88 Example 3 0.71 Example 8 0.94 Example 4 0.99 Example 9 0.89 Example 5 0.99

[0074] The N / P removal molar ratio is the proportion of the number of moles of ammonia nitrogen and phosphate removed after step S2 to the number of moles of ammonia nitrogen and phosphate before the reaction. The closer the N / P removal molar ratio is to 1, the more thorough the removal of ammonia nitrogen and phosphate.

[0075] The table above shows that the concentrated solution containing ammonia nitrogen contains humic acid and Cl... - CO3 2- When the concentration is within the specified range, the ferrous ammonium phosphate generated in step S2 has a higher purity, which is beneficial for the preparation of high-purity iron phosphate in step S3. However, calcium and magnesium ions have an adverse effect on the reaction in step S2.

[0076] Example 10

[0077] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the molar ratio of iron, phosphorus, and nitrogen in the reactor in step S2 is 1:1:1.

[0078] Example 11

[0079] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that the molar ratio of iron, phosphorus, and nitrogen in the reactor in step S2 is 0.9:0.9:1.

[0080] Example 12

[0081] The method for recovering and utilizing nitrogen and phosphorus resources from wastewater described in this embodiment is the same as that in Embodiment 1, except that, as Figures 1-3 As shown, the tubular furnace in step S3 includes a furnace tube 1 and a sample rack 2 inside the furnace tube 1. The furnace tube 1 is a hollow cylinder and is horizontal. An oxygen inlet 3 is provided at the lower part of one end of the furnace tube 1, and a gas outlet 4 is provided at the other end to discharge the generated ammonia and excess oxygen.

[0082] One end of the sample holder 2 extends out of the furnace tube 1 and is connected to a drive motor outside the furnace tube 1 to drive the sample holder 2 to rotate. The sample holder 2 includes five sample trays 5 and four support rods 6. The five sample trays 5 are evenly distributed along the axial direction of the furnace tube 1 and are concentric with the furnace tube 1. The support rods 6 are parallel to the axial direction of the furnace tube 1 and are evenly distributed along the circumference of the sample trays 5. The support rods 6 surround the edge of the sample trays 5 and connect to the edge of the sample trays 5, so that several sample trays 5 are connected in a string.

[0083] The sample tray 5 is a flat cylinder with mesh on its outer side walls, which allows oxygen to enter and exit the sample tray 5 while preventing solid materials inside the sample tray 5 from leaking out.

[0084] The circular surface of the sample tray 5 is perpendicular to the axis of the furnace tube 1. The sample tray 5 is equipped with several horizontal and vertically intersecting partition plates 7, which divide the internal space of the sample tray 5 into several grid-like sub-spaces 8, so that the sample can be evenly distributed in the sample tray 5.

[0085] The sample tray 5 facing the oxygen inlet 3 has a uniform wavy mesh, which helps to disturb the oxygen on the upstream side of the sample tray 5 and promote gas-solid contact.

[0086] The sample tray 5 has a surface area smaller than the longitudinal section inside the furnace tube 1. When the sample holder 2 is not rotating, the four support rods 6 are connected to the upper, lower, left, and right points of the edge of the sample tray 5 respectively. Starting from the sample tray 5 closest to the oxygen inlet 3, the first sample tray 5 is not connected to the upper support rod 6 only at the top, the second sample tray 5 is not connected to the right support rod 6 only at the right side, the third sample tray 5 is not connected to the lower support rod 6 only at the bottom, the fourth sample tray 5 is not connected to the left support rod 6 only at the left side, and the fifth to eighth sample trays 5 repeat the connection pattern of the first to fourth sample trays 5.

[0087] In step S2, a clean sample holder 2 is placed inside the reaction vessel 9 and used as a stirrer. One end of the sample holder 2 extends upward through the reaction vessel 9 and is connected to a stirring motor to drive the sample tray 5 to rotate. The sample tray 5 is placed horizontally with its corrugated surface facing upward. An aeration device is provided at the bottom of the reaction vessel to provide upward agitation to the lower and middle parts of the reaction vessel.

[0088] Table 2 Comparison of the effects of Examples 1, 10-12 and Comparative Example 1

[0089] Purity (%) of ferric phosphate product Example 1 95.8 Example 10 94.3 Example 11 90.2 Example 12 97.4 Comparative Example 1 74.3

[0090] As shown in the table above, the method for recovering and utilizing nitrogen and phosphorus resources from wastewater provided by the present invention involves concentration followed by reaction with phosphate and ferrous salts to generate ferrous ammonium phosphate crystals, which are then pyrolyzed in an oxygen atmosphere to generate ferric phosphate and ammonia. This method achieves simultaneous recovery of ammonia nitrogen and phosphorus from wastewater and yields ferric phosphate with high purity.

Claims

1. A method for recovering and utilizing nitrogen and phosphorus resources from wastewater, characterized in that, Includes the following steps: S1: Concentrate the wastewater to increase the concentration of ammonia nitrogen in the wastewater, which will facilitate subsequent reactions; S2: Add the concentrated solution containing ammonia nitrogen to the reaction vessel, then add phosphate salt and ferrous salt to carry out the crystallization reaction and generate the double salt ferrous ammonium phosphate. S3: Ferrous ammonium phosphate is added to a pyrolysis furnace and pyrolyzed under an oxygen atmosphere to obtain ferric phosphate; S4: Collect the ammonia gas discharged from the pyrolysis furnace, then absorb it with acid to produce liquid nitrogen fertilizer; The tubular furnace used in step S3 includes a furnace tube and a sample rack inside the furnace tube. The furnace tube is a hollow cylinder and is horizontal. An oxygen inlet is provided at the lower part of one end of the furnace tube, and a gas outlet is provided at the other end to discharge the generated ammonia and excess oxygen. One end of the sample rack extends out of the furnace tube and is connected to a drive motor outside the furnace tube to drive the sample rack to rotate. The sample rack includes several sample trays and several support rods. The sample trays are evenly distributed along the axial direction of the furnace tube, and the support rods are parallel to the axial direction of the furnace tube. The sample tray is a flat cylindrical shape, and its outer walls are all made of mesh, which allows oxygen to enter and exit the sample tray while preventing solid materials inside the sample tray from leaking out. The circular surface of the sample tray is perpendicular to the axis of the furnace tube. The sample tray is equipped with several intersecting partitions, which divide the internal space of the sample tray into several grid-like sub-spaces, so as to facilitate the uniform distribution of the sample in the sample tray. The sample tray facing the oxygen inlet has a uniform corrugated mesh, which helps to agitate the oxygen on the upstream side of the sample tray and promote gas-solid contact. In step S1, the wastewater is concentrated using a two-stage electrodialysis method, specifically including the following steps: (1) Raw material wastewater is fed into the first-stage electrodialysis unit for electrodialysis. The concentrated wastewater generated in the concentrated water chamber of the first-stage electrodialysis unit is collected, and the fresh water generated in the fresh water chamber is used as recycled water. (2) The concentrated wastewater generated in step (1) is fed into a secondary electrodialysis device for electrodialysis to reduce the hardness of the concentrated wastewater and obtain the concentrated liquid containing ammonia nitrogen. In step S2, the concentration of humic acid in the concentrated solution containing ammonia nitrogen is 300-500 mg / L, and Cl... - The concentration is 700-1000 mg / L, CO3 2- The concentration is 50-70 mg / L.

2. The method for recovering and utilizing nitrogen and phosphorus resources from wastewater according to claim 1, characterized in that, The secondary electrodialysis device is equipped with a cathode plate, a cation exchange membrane, and an anode plate in sequence. The cations in the concentrated wastewater enter the chamber between the cation exchange membrane and the cathode plate, while the concentrated wastewater with reduced hardness remains in the chamber between the cation exchange membrane and the anode plate. The concentration of ammonia nitrogen in the concentrated solution containing ammonia nitrogen is not less than 5.6 g / L.

3. The method for recovering and utilizing nitrogen and phosphorus resources from wastewater according to claim 1, characterized in that, In step S2, the phosphate salt is selected from hydrogen phosphate or dihydrogen phosphate, and the ferrous salt is selected from ferrous chloride, ferrous sulfate, or ferrous nitrate.

4. The method for recovering and utilizing nitrogen and phosphorus resources from wastewater according to claim 1, characterized in that, In step S2, the molar ratio of iron, phosphorus, and nitrogen in the reactor is (1-2):(1-2):

1. The pH of the reaction solution is controlled between 8 and 10 using NaOH or KOH. The reaction is carried out at 60-80℃ for 0.5-2.0 h, and the stirring rate is controlled at 60-120 rpm.

5. The method for recovering and utilizing nitrogen and phosphorus resources from wastewater according to claim 1, characterized in that, In step S3, the product from step S2 is filtered and sent to a pyrolysis furnace, oxygen is introduced, the temperature is raised at a rate of 8-10 °C / min, the temperature is raised to 800-900 °C and maintained for 30-50 min, and then cooled to obtain the final product, iron phosphate.

6. The method for recovering and utilizing nitrogen and phosphorus resources from wastewater according to claim 1, characterized in that, The sample tray is concentrically arranged with the furnace tube, and several support rods are evenly distributed along the circumference of the sample tray. The support rods surround the edge of the sample tray and connect to the edge of the sample tray, so that several sample trays are connected in a series.

7. The method for recovering and utilizing nitrogen and phosphorus resources from wastewater according to claim 6, characterized in that, In step S2, a clean sample holder is placed inside the reaction vessel and used as a stirrer. One end of the sample holder extends upward through the reaction vessel and is connected to a stirring motor to drive the sample tray to rotate. The sample tray is placed horizontally with the corrugated side facing upwards.

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