A method for treating iron phosphate wastewater

Abstract

The application discloses a treatment method of ferric phosphate wastewater, comprising the following steps: S1. adding a certain amount of water-soluble ferrous salt and an oxidizing agent into the ferric phosphate wastewater, so that the molar ratio of phosphorus to iron in the wastewater is 1:(1-5); S2. neutralizing by adding alkali, and adjusting the pH to 2.5-3.5; S3. performing solid-liquid separation to obtain a filter cake and a filtrate; S4. firstly performing ceramic membrane separation on the filtrate, then performing nanofiltration membrane treatment on the ceramic membrane clear liquid, and then performing reverse osmosis membrane treatment on the nanofiltration membrane clear liquid to obtain a reverse osmosis clear liquid and a reverse osmosis concentrated liquid; and S5. mixing the nanofiltration membrane concentrated liquid and the reverse osmosis concentrated liquid, and then performing treatment by using an MVR system to obtain an MVR evaporation liquid and a sulfate crystal product. The application is not introduced with calcium ions, and is treated by using a one-step precipitation method under a low pH condition, and is comprehensively treated by using a ceramic membrane + nanofiltration membrane + reverse osmosis membrane, so that the purpose of zero discharge of the ferric phosphate wastewater is achieved.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, and specifically relates to a method for treating ferric phosphate wastewater. Background Technology

[0002] The production of ferric phosphate generates a large amount of acidic wastewater with a phosphorus content of approximately 700–1500 mg / L and a sulfate content of approximately 10–90 g / L. Currently, the common treatment method involves neutralizing the wastewater with lime before it enters a membrane treatment system. After treatment in the membrane system, it then enters an MVR (Medium-Vacuum Reduction) system for crystallization and concentration. This treatment method reduces the sulfate content during the calcium-based process. However, because calcium ions are introduced into the system, their impact on membrane treatment must be eliminated before membrane treatment. Therefore, the overall process for treating ferric phosphate wastewater is quite complex, and the presence of calcium ions poses a potential risk of scaling in the membrane treatment system.

[0003] Patent CN202211050148.7 discloses a process for the resource-based treatment of ferric phosphate mother liquor produced by oxidation. This patent first adds an oxidant and ferrous salt to the ferric phosphate mother liquor, then gradually adjusts the pH. The first step adjusts the pH to 1.8–2.0, obtaining ferric phosphate filter cake and primary filtrate. The pH of the primary filtrate is then adjusted to 1.0–5.0, obtaining ferric hydroxide filter cake and secondary filtrate. The secondary filtrate is then sequentially treated by ultrafiltration and a membrane concentration system (two-stage reverse osmosis). The concentrate is then extracted, evaporated, and crystallized to obtain ammonium sulfate, manganese sulfate, and magnesium sulfate products. This patent achieves the resource-based treatment of ferric phosphate mother liquor without using calcium ions. However, this patent also has certain problems in practical applications: First, during the gradual pH adjustment for precipitation, ferric phosphate cannot precipitate completely under conditions of 1.8–2.0, resulting in the formation of ferric hydroxide precipitate containing a certain amount of ferric phosphate, affecting its recycling. Second, after ultrafiltration, the filtrate or wash water obtained during the ferric phosphate production process has a high salt content and therefore high conductivity. Ultrafiltration membranes can only filter suspended solids in water and cannot remove high-valence ions. Subsequent reverse osmosis concentration must be high-pressure reverse osmosis concentration. The increased pressure places higher demands on the membrane modules and related accessories. The higher osmotic pressure also results in lower effluent flow and higher energy consumption. Therefore, a new method for treating ferric phosphate wastewater is needed. Summary of the Invention

[0004] The purpose of this invention is to provide a method for treating ferric phosphate wastewater in order to overcome the shortcomings of the prior art.

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

[0006] A method for treating ferric phosphate wastewater includes the following steps:

[0007] S1. Add a certain amount of water-soluble ferrous salt and oxidant to the ferric phosphate wastewater so that the phosphorus-iron molar ratio in the wastewater is 1:(1-5), wherein the amount of oxidant added is sufficient to oxidize all the ferrous salt in the wastewater to ferric iron; or directly add ferric soluble salt to the wastewater so that the phosphorus-iron molar ratio in the wastewater is 1:(1-5).

[0008] S2. Neutralize with alkali, adjust the pH to 2.5-3.5, and produce ferric phosphate precipitate;

[0009] S3. After solid-liquid separation, a filter cake containing ferric phosphate precipitate and a filtrate containing sulfate ions are obtained;

[0010] S4. The filtrate is first separated by a ceramic membrane to remove solid particles, resulting in a clear ceramic membrane solution. Then, the clear ceramic membrane solution is treated by a nanofiltration membrane to retain phosphate and sulfate ions, resulting in a nanofiltration membrane concentrate containing most sulfate and phosphate ions and a clear nanofiltration membrane solution containing a small amount of sulfate and phosphate ions. The clear nanofiltration membrane solution is then treated by a reverse osmosis membrane to obtain a reverse osmosis solution that is basically free of phosphate and sulfate ions and a reverse osmosis concentrate containing most sulfate and phosphate ions.

[0011] S5. The nanofiltration membrane concentrate and the reverse osmosis concentrate are mixed and then processed using an MVR system to obtain MVR evaporator and sulfate crystal products.

[0012] Preferably, the phosphorus content in the ferric phosphate wastewater is 700–1500 mg / L, and the sulfate content is 10–90 g / L.

[0013] Preferably, the ferrous sulfate in step S1 is added in the form of solid or solution.

[0014] Preferably, the oxidant in step S1 is hydrogen peroxide, and the amount of hydrogen peroxide used is 0.4 to 0.6 times the molar amount of ferrous ions in the wastewater.

[0015] Preferably, in step S1, ferrous iron is first added, and the reaction is stirred for 2 to 8 hours, followed by the addition of an oxidant, and the reaction is carried out for 0.5 to 1.5 hours.

[0016] Preferably, the filter cake after solid-liquid separation in step S3 is mixed evenly with the ferric phosphate slurry produced normally, then subjected to pressure filtration and water washing, and then calcined to obtain the ferric phosphate product.

[0017] Preferably, the ceramic membrane in step S4 has a pore size of 10–100 nm, and the nanofiltration membrane has a pore size of 150–800 D.

[0018] This application provides a novel method for treating ferric phosphate wastewater. Without introducing calcium ions, it employs a one-step precipitation method under low pH conditions, and integrates ceramic membrane, nanofiltration membrane, and reverse osmosis membrane treatment to achieve zero discharge of ferric phosphate wastewater. This simplifies the existing ferric phosphate wastewater treatment process and eliminates the risk of scaling caused by calcium ions in the membrane treatment system. Detailed Implementation

[0019] This application provides a method for treating ferric phosphate wastewater, comprising the following steps:

[0020] S1. Add a certain amount of water-soluble ferrous salt and oxidant to the ferric phosphate wastewater so that the phosphorus-iron molar ratio in the wastewater is 1:(1-5). The amount of oxidant added should be sufficient to oxidize all the ferrous salt in the wastewater to ferric iron; or add ferric soluble salt directly to the wastewater so that the phosphorus-iron molar ratio in the wastewater is 1:(1-5).

[0021] The ferric phosphate wastewater treated in this application generally has a phosphorus content of 700–1500 mg / L and a sulfate content of 10–90 g / L. First, ferrous ions and an oxidant are added, or trivalent soluble ferric salts are added directly. This allows the phosphate and trivalent iron in the wastewater to react and form ferric phosphate precipitate, thereby removing most of the phosphate in the wastewater. In order to reduce the introduction of other impurities when adding ferrous ions, ferrous ions are preferably added in the form of solid or solution of ferrous sulfate, and trivalent iron is preferably added in the form of solid or solution of ferric sulfate.

[0022] Experiments have shown that when soluble ferric salts are added directly, the pH of the system decreases further due to the hydrolysis of ferric iron itself, which leads to an increase in the amount of alkali used during subsequent neutralization. Therefore, it is preferable to add water-soluble ferrous salts and oxidants.

[0023] When adding ferrous ions and oxidant, it is preferable to add ferrous ions first and stir the reaction for 2 to 8 hours to allow ferrous ions and phosphate ions to fully react and generate ferrous phosphate. Then, add the oxidant and react for 0.5 to 1.5 hours to generate ferric iron from ferrous ions. Since ferric phosphate wastewater contains a certain amount of other impurities, adding the oxidant first would increase the amount of oxidant required. Therefore, it is preferable to add ferrous ions first.

[0024] Oxidizing agents can be hydrogen peroxide, sodium hypochlorite, etc., with hydrogen peroxide being preferred. The ratio of the amount of hydrogen peroxide to the molar amount of ferrous ions in the wastewater is 0.4 to 0.6:1.

[0025] S2. Neutralize with alkali and adjust the pH to 2.5-3.5 to produce ferric phosphate precipitate; sodium hydroxide is preferred as the alkali; this application directly adjusts the pH to 2.5-3.5, which, compared with the two-step precipitation of the prior art, allows ferric phosphate to precipitate as much as possible under pH 2.5-3.5 conditions, while ferric hydroxide will hardly precipitate.

[0026] S3. Solid-liquid separation is preferably carried out by pressure filtration to obtain a filter cake containing ferric phosphate precipitate and a filtrate containing sulfate ions. The phosphate ions in the wastewater cannot all be converted into ferric phosphate precipitate after the above treatment. In addition to sulfate ions, the filtrate also contains a small amount of phosphate ions, and the phosphorus ion content in the filtrate is reduced to 0-100 mg / L.

[0027] Inevitably, some sulfate ions and trace amounts of ferric hydroxide precipitate remain in the ferric phosphate filter cake. Normally, water washing is used to remove the salt from the filter cake. This application preferably involves uniformly mixing the solid-liquid separated ferric phosphate filter cake with a normally produced ferric phosphate slurry (obtained using conventional methods such as the ammonium method or sodium method, with a pH generally between 2 and 4). This allows the residual sulfate ions in the filter cake to redissolve, and the acidity of the normally produced ferric phosphate slurry can also dissolve the ferric hydroxide precipitate in the filter cake. After pressure filtration and water washing, the residual sulfate and ferric hydroxide content in the filter cake is reduced. Calcination yields a high-purity ferric phosphate product, thus preventing fluctuations in product quality. Using this method, ferric phosphate can be recovered, and salt and ferric hydroxide precipitate removed from the filter cake can be removed without requiring additional equipment.

[0028] S4. Even after the solid-liquid separation in step S3, some iron phosphate particles and other solid particles will still remain in the filtrate. Therefore, this application first separates the filtrate through a ceramic membrane to remove solid particles, obtaining a clear ceramic membrane solution. Then, the clear ceramic membrane solution is treated with a nanofiltration membrane to retain phosphate and sulfate ions, obtaining a nanofiltration membrane concentrate containing most sulfate and phosphate ions and a nanofiltration membrane clear solution containing a small amount of sulfate and phosphate ions. The phosphorus content in the nanofiltration membrane concentrate is approximately 0–500 mg / L, and the sulfate content is approximately 10–17 mg / L. The phosphorus content in the nanofiltration membrane supernatant is approximately 0–100 mg / L, and the sulfate content is approximately 0–10 g / L. The nanofiltration membrane supernatant is then treated with a reverse osmosis membrane to further remove phosphate and sulfate ions, yielding a reverse osmosis supernatant that is essentially free of phosphate and sulfate ions, and a reverse osmosis concentrate that contains most sulfate and phosphate ions. The reverse osmosis concentrate contains approximately 0–500 mg / L of phosphorus and approximately 0–30 g / L of sulfate. The reverse osmosis supernatant, being essentially free of phosphate and sulfate ions, can be reused as wash water in other processes.

[0029] This application replaces the existing ultrafiltration + reverse osmosis technology with ceramic membrane filtration and nanofiltration + reverse osmosis membrane filtration. Firstly, compared to ultrafiltration membranes, ceramic membranes, in addition to removing suspended solids from water, also exhibit superior resistance to high temperatures, acids, and alkalis. Furthermore, the filter cartridges have a longer lifespan and are easier to replace, making them more suitable for pre-filtration. Following ceramic membrane filtration, this application uses nanofiltration membranes. Nanofiltration membranes can retain a certain amount of high-valence ions, therefore, the downstream reverse osmosis can be performed using ordinary reverse osmosis at atmospheric pressure, while simultaneously ensuring the quality of the reverse osmosis effluent.

[0030] S5. The nanofiltration membrane concentrate and the reverse osmosis concentrate are mixed and then treated using an MVR system to obtain an MVR evaporator that is essentially free of phosphate and sulfate ions, and a pure sulfate crystal product. The MVR evaporator is reused as wash water in other processes, and the sulfate product can be reused in the chlor-alkali workshop, enabling the separation and recovery of all elements in the ferric phosphate wastewater, achieving the goal of zero discharge of ferric phosphate wastewater.

[0031] Therefore, this application provides a novel method for treating ferric phosphate wastewater. Without introducing calcium ions, it employs a one-step precipitation method and integrates ceramic membrane, nanofiltration membrane, and reverse osmosis membrane treatment to achieve zero discharge of ferric phosphate wastewater. This simplifies the existing ferric phosphate wastewater treatment process and eliminates the risk of scaling caused by calcium ions in the membrane treatment system.

[0032] Preferably, in step S4, the pore size of the ceramic membrane is 10–100 nm, and the pore size of the nanofiltration membrane is 150–800 D.

[0033] Example 1

[0034] 1. Add a certain amount of ferrous sulfate solid to the ferric phosphate wastewater and stir. The phosphorus content in the ferric phosphate wastewater is 718 mg / L, and the amount of ferrous sulfate added is to make the ferrous concentration in the wastewater 6.4 g / L. The stirring time is 3 hours.

[0035] 2. After stirring, add hydrogen peroxide to the reaction system and stir for 1 hour. The amount of hydrogen peroxide added is to make the hydrogen peroxide content in the wastewater reach 1.5 g / L.

[0036] 3. After stirring, add NaOH to the system to control the final pH of neutralization to 2.8, and stir until homogeneous;

[0037] 4. The neutralized system was subjected to pressure filtration to obtain filter cake and filtrate. The filter cake sample was washed with water and tested. Its iron content was 36.6%, phosphorus content was 20.7%, and iron-phosphorus molar ratio was 0.980. The results of elemental analysis of impurities in the filter cake after water washing are shown in Table 1. The phosphorus content in the filtrate was 38 mg / L and sulfate content was 21 g / L.

[0038] The filter cake was mixed with the slurry (pH 2) of ferric phosphate before normal production, and then filtered and washed with water. The iron content in the washed filter cake after mixing was measured to be 35.9%, the phosphorus content was 20.1%, the iron-phosphorus ratio was 0.996, and the content of impurity elements was basically the same as in Table 1. After calcination, ferric phosphate product was obtained.

[0039] Table 1

[0040] Al(％) Ca (%) Cu (%) Mg (%) Mn(%) Na (%) Ti (%) Zn(%) S(％) 0.003 0.003 0.001 0.003 0.007 0.006 0.007 0.003 0.007

[0041] 5. The filtrate is passed through a ceramic membrane with a pore size of 50 nm to remove solid iron phosphate. The clear liquid from the ceramic membrane is then processed through a nanofiltration membrane with a pore size of 400 D to obtain nanofiltration concentrate and nanofiltration clear liquid. The nanofiltration concentrate contains 156 mg / L of phosphorus and 89 g / L of sulfate, while the nanofiltration clear liquid contains 1.78 mg / L of phosphorus and 2.2 g / L of sulfate. The nanofiltration clear liquid is then transferred to a reverse osmosis membrane for further processing to obtain reverse osmosis concentrate and reverse osmosis clear liquid. The reverse osmosis clear liquid contains 0.10 mg / L of phosphorus and 0.13 g / L of sulfate, while the reverse osmosis concentrate contains 210 mg / L of phosphorus and 20.3 g / L of sulfate.

[0042] 6. The nanofiltration membrane concentrate and the reverse osmosis membrane concentrate were mixed and then fed into the MVR section for concentration to obtain MVR evaporate and sodium sulfate crystals. The elements in the MVR evaporate and sodium sulfate crystals were analyzed, and the results are shown in Tables 2 and 3, respectively.

[0043] Table 2

[0044] Ammonia nitrogen (mg / L) COD (mg / L) Sulfate (mg / L) Na (mg / L) 0.56 15 56 14

[0045] Table 3

[0046] <![CDATA[Na2SO4(%)]]> <![CDATA[Na3PO4(%)]]> 99.8 0.2

[0047] Example 2

[0048] 1. Add a certain amount of ferrous sulfate solid to the ferric phosphate wastewater and stir. The phosphorus content in the ferric phosphate wastewater is 1238 mg / L, and the amount of ferrous sulfate added is to make the ferrous concentration in the wastewater 8.83 g / L. The stirring time is 3 hours.

[0049] 2. After stirring, add hydrogen peroxide to the reaction system and stir for 1 hour. The amount of hydrogen peroxide added is to make the hydrogen peroxide content in the wastewater reach 2.0 g / L.

[0050] 3. After stirring, add NaOH to the system to control the final pH of neutralization to 3.2, and stir evenly.

[0051] 4. The neutralized system was subjected to pressure filtration to obtain filter cake and filtrate. The filter cake sample was washed with water and tested. Its iron content was 36.5%, phosphorus content was 20.6%, and iron-phosphorus ratio was 0.978. The results of elemental analysis of impurities in the filter cake after water washing are shown in Table 4. The phosphorus content in the filtrate was 25 mg / L and the sulfate content was 29 g / L.

[0052] The filter cake was mixed with the slurry (pH 2) of ferric phosphate before normal production, and then filtered and washed with water. The iron content in the washed filter cake after mixing was measured to be 36.2%, the phosphorus content was 20.08%, the iron-phosphorus ratio was 1.0, and the content of impurity elements was basically the same as in Table 4. After calcination, ferric phosphate product was obtained.

[0053] Table 4

[0054] Al(％) Ca (%) Cu (%) Mg (%) Mn(%) Na (%) Ti (%) Zn(%) S(％) 0.002 0.003 0.001 0.002 0.006 0.006 0.007 0.003 0.005

[0055] 5. The filtrate is passed through a ceramic membrane with a pore size of 50 nm to remove solid iron phosphate. The clear liquid from the ceramic membrane is then processed through a nanofiltration membrane with a pore size of 400 D to obtain nanofiltration concentrate and nanofiltration clear liquid. The nanofiltration concentrate contains 102 mg / L of phosphorus and 110 g / L of sulfate, while the nanofiltration clear liquid contains 0.89 mg / L of phosphorus and 1.6 g / L of sulfate. The nanofiltration clear liquid is then transferred to a reverse osmosis membrane for further processing to obtain reverse osmosis concentrate and reverse osmosis clear liquid. The reverse osmosis clear liquid contains 0.05 mg / L of phosphorus and 0.53 g / L of sulfate, while the reverse osmosis concentrate contains 210 mg / L of phosphorus and 20.3 g / L of sulfate.

[0056] 6. The nanofiltration membrane concentrate and the reverse osmosis membrane concentrate were mixed and then fed into the MVR section for concentration to obtain MVR evaporate and sodium sulfate crystals. The elements in the MVR evaporate and sodium sulfate crystals were analyzed, and the results are shown in Tables 5 and 6, respectively.

[0057] Table 5

[0058] Ammonia nitrogen (mg / L) COD (mg / L) Sulfate (mg / L) Na (mg / L) 0.18 13 42 21

[0059] Table 6

[0060] <![CDATA[Na2SO4(%)]]> <![CDATA[Na3PO4(%)]]> 99.87 0.13

[0061] Comparative Example 1

[0062] 1. Add a certain amount of ferrous iron to the ferric phosphate wastewater and stir. The phosphorus content in the wastewater is 718 mg / L, the concentration of ferrous iron added is 6.4 g / L, and the stirring time is 3 hours.

[0063] 2. After stirring, add NaOH to the system to control the final pH of neutralization to 2.8, and stir evenly.

[0064] 3. The neutralized system was subjected to pressure filtration to obtain filter cake and filtrate; the phosphorus content in the filtrate was 650 mg / L and the sulfate content was 21 g / L; this proved that without the addition of an oxidant, under low pH conditions, ferrous phosphate generated from phosphate ions and ferrous ions is not easily precipitated and removed, therefore a large amount of phosphate ions still exist in the filtrate.

[0065] 4. The filtrate enters a ceramic membrane with a pore size of 50 nm to remove the solid iron phosphate. The clear liquid from the ceramic membrane then enters a nanofiltration membrane with a pore size of 400 D to obtain nanofiltration concentrate and nanofiltration clear liquid. The nanofiltration concentrate contains 785 mg / L of phosphorus and 35 g / L of sulfate, while the nanofiltration clear liquid contains 632 mg / L of phosphorus and 11.3 g / L of sulfate. The nanofiltration clear liquid is then transferred to a reverse osmosis membrane for further processing to obtain reverse osmosis concentrate and reverse osmosis clear liquid. The reverse osmosis concentrate contains 756 mg / L of phosphorus and 20.3 g / L of sulfate, while the reverse osmosis clear liquid contains 618 mg / L of phosphorus and 13.8 g / L of sulfate. The membrane treatment system cannot separate sulfate and phosphate, resulting in phosphate loss.

[0066] Comparative Example 2

[0067] 1. Add a certain amount of lime to the ferric phosphate wastewater and stir to adjust the pH to 2.1. The phosphorus content in the ferric phosphate wastewater is 718 mg / L. The stirring time is 3 hours.

[0068] 2. After stirring, the calcium content in the system was 289 mg / L, and the pH was adjusted to 10.85 using NaOH.

[0069] 3. After the reaction was completed, the mixture was filtered under pressure. The phosphorus content in the filtrate was 13.2 mg / L and the calcium content was 198 mg / L.

[0070] 4. After adjusting the pH to neutral using sulfuric acid, the solution enters a ceramic membrane to remove solid impurities and then enters reverse osmosis. However, due to the high calcium content in the filtrate, the operating pressure of the reverse osmosis membrane becomes too high, making it impossible to continue membrane filtration.

[0071] Comparative Example 3

[0072] 1. Add a certain amount of ferrous sulfate solid to the ferric phosphate wastewater and stir. The phosphorus content in the ferric phosphate wastewater is 1238 mg / L, and the amount of ferrous sulfate added is to make the ferrous concentration in the wastewater 8.83 g / L. The stirring time is 3 hours.

[0073] 2. After stirring, add hydrogen peroxide to the reaction system and stir for 1 hour. The amount of hydrogen peroxide added is to make the hydrogen peroxide content in the wastewater reach 2.0 g / L.

[0074] 3. After stirring, add NaOH to the system to control the final pH of neutralization to 4.0, and stir until homogeneous;

[0075] 4. The neutralized system was subjected to pressure filtration to obtain a filter cake and filtrate. The filter cake sample was washed with water and analyzed; its iron content was 38.8%, phosphorus content was 19.9%, and the iron-to-phosphorus ratio was 1.076, which does not meet the requirements for iron phosphate content in batteries. This proves that the neutralization pH > 3.5, resulting in the formation of iron hydroxide precipitate, which reduces the purity of iron phosphate. The filtrate contained 25 mg / L of phosphorus and 29 g / L of sulfate.

[0076] Comparative Example 4

[0077] 1. Add a certain amount of ferrous sulfate solid to the ferric phosphate wastewater and stir. The phosphorus content in the ferric phosphate wastewater is 1238 mg / L, and the amount of ferrous sulfate added is to make the ferrous concentration in the wastewater 8.83 g / L. The stirring time is 3 hours.

[0078] 2. After stirring, add hydrogen peroxide to the reaction system and stir for 1 hour. The amount of hydrogen peroxide added is to make the hydrogen peroxide content in the wastewater reach 2.0 g / L.

[0079] 3. After stirring, add NaOH to the system to control the final pH of neutralization to 5.0, and stir until homogeneous;

[0080] 4. The neutralized system was pressure filtered to obtain a filter cake and a filtrate. The phosphorus content in the filtrate was 25 mg / L, and the sulfate content was 29 g / L. The filter cake was mixed with the ferric phosphate slurry before normal production filtration and washed again with water. The iron content in the filter cake was measured to be 39.5%, the phosphorus content was 18.9%, and the iron-to-phosphorus ratio was 1.16, which does not meet the requirements for ferric phosphate content in batteries. Compared with Comparative Example 3, this comparative example had a higher neutralization pH and more ferric hydroxide precipitate.

[0081] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if these modifications and modifications of the invention fall within the scope of the claims and their equivalents, the invention is also intended to include these modifications and modifications.

Claims

1. A method for treating iron phosphate wastewater, characterized by, The method comprises the following steps: S1. Adding a certain amount of water-soluble ferrous salt and oxidizing agent to the ferric phosphate wastewater, so that the molar ratio of phosphorus to iron in the wastewater is 1:(1-5), and the amount of the oxidizing agent is added according to the amount of the ferrous ion in the wastewater so that all the ferrous ions are oxidized to ferric ions; or directly adding a soluble ferric salt to the wastewater so that the molar ratio of phosphorus to iron in the wastewater is 1:(1-5); the phosphorus content in the ferric phosphate wastewater is 700-1500 mg / L, and the sulfate content is 10-90 g / L; the ferrous salt is added in the form of ferrous sulfate solid or solution, and the soluble ferric salt is added in the form of ferric sulfate solid or solution; S2. Neutralizing by adding alkali to adjust the pH to 2.5-3.5 to produce ferric phosphate precipitate; S3. Solid-liquid separation to obtain a filter cake containing ferric phosphate precipitate and a filtrate containing sulfate ions; S4. First, the filtrate is separated by a ceramic membrane to remove solid particles to obtain a ceramic membrane clear liquid; then, the ceramic membrane clear liquid is treated by a nanofiltration membrane to retain phosphate and sulfate ions, to obtain a nanofiltration membrane concentrate containing most of the sulfate ions and phosphate ions, and a nanofiltration membrane clear liquid containing a small amount of sulfate ions and phosphate ions; and then, the nanofiltration membrane clear liquid is treated by a reverse osmosis membrane to obtain a reverse osmosis clear liquid containing substantially no phosphate and sulfate ions, and a reverse osmosis concentrate containing most of the sulfate ions and phosphate ions; S5. Mixing the nanofiltration membrane concentrate and the reverse osmosis concentrate, and then treating by an MVR system to obtain MVR evaporation liquid and sulfate salt crystal product; In step S1, the ferrous salt is first added, and after stirring for 2-8 h, the oxidizing agent is added, and the reaction is carried out for 0.5-1.5 h.

2. The method for treating ferric phosphate wastewater according to claim 1, characterized in that, The oxidizing agent in step S1 is hydrogen peroxide, and the amount of the hydrogen peroxide is 0.4-0.6:1 compared with the molar amount of the ferrous ions in the wastewater.

3. The method for treating ferric phosphate wastewater according to claim 1, characterized in that, After the filter cake obtained after the solid-liquid separation in step S3 is mixed uniformly with the normal production ferric phosphate slurry, water washing is carried out by pressure filtration, and then calcination is carried out to obtain ferric phosphate product.

4. The method for treating ferric phosphate wastewater according to claim 1, characterized in that, The pore size of the ceramic membrane in step S4 is 10-100 nm, and the pore size of the nanofiltration membrane is 150-800 D.