A method for treating iron phosphate production wastewater

Abstract

The application discloses a kind of phosphorus iron production wastewater treatment methods, belong to phosphorus iron material production wastewater treatment method technical field.The steps include: S1: evaporation;S2: first reuse;S3: second reuse;S4: neutralization.Through the concentration of phosphoric acid and metal impurity ions obtained in each step of impurity removal operation, the effect of impurity removal and reuse is good, and according to the actual situation of metal impurity ions in the two concentrated water obtained by impurity removal, different wastewater treatment methods are used, which can better remove the metal impurity ions that are difficult to remove, realize the recycling of phosphoric acid, and make the reuse liquid suitable for the production process of phosphorus iron, efficient impurity removal and efficient reuse.

Description

Technical Field

[0001] This invention belongs to the field of battery material production technology, and more specifically, relates to the technical field of wastewater treatment methods for iron phosphate material production. Background Technology

[0002] With the rapid development of power batteries and energy storage lithium batteries, the existing battery capacity is gradually failing to meet the demands of the public. Lithium iron phosphate (LFP), as a lithium-ion battery cathode material with good electrical performance and relatively stable properties, has a very large future market potential. Its production process mainly uses solid-state methods, which can be divided into iron oxide red method, iron oxalate method, and iron phosphate method according to the different iron sources. Among them, the iron phosphate process, as the most recently developed process, has gradually become the mainstream process due to its advantages of simplicity, low energy consumption, and high sintering rate. Because lithium iron phosphate and iron phosphate have a very similar structure, lithium iron phosphate crystal particles can be directly grown on the basis of iron phosphate particles. Therefore, the preparation of the precursor iron phosphate is extremely important in this route.

[0003] Currently, the mainstream production routes for ferric phosphate are the "ammonium method" (ferrous sulfate + monoammonium phosphate) and the "sodium method" (phosphoric acid + liquid alkali + ferrous sulfate). The "iron method" (phosphoric acid + iron powder) is also gaining popularity. Although the raw material price is higher, its waste product is mainly hydrogen gas, resulting in less wastewater and less environmental pressure. As safety and environmental protection pressures gradually increase, it will become a mainstream process. In existing "iron method" ferric phosphate production processes, iron powder and phosphoric acid are mainly used as raw materials to prepare ferric phosphate. However, to reduce the production cost of ferric phosphate, existing technologies often choose to use ferrous sulfate, a byproduct of titanium dioxide production, as the iron source for ferric phosphate production. This is also of great significance for maximizing resource utilization and achieving a circular economy. In the titanium dioxide industry, when using large electric furnaces and titanium concentrate and coke as raw materials to produce high-titanium slag, molten iron is generated as a byproduct. After high-temperature deoxidation, desulfurization, and decarburization processes, pig iron can be produced. Statistics show that one ton of titanium slag produces approximately 400 kilograms of pig iron as a byproduct. It is estimated that my country produces about 800,000 tons of pig iron annually. Therefore, the output of pig iron byproducts from titanium slag is relatively large and the sources are wide. Moreover, this pig iron has low content of elements other than carbon and sulfur. If the pig iron byproduct of titanium slag in the titanium industry can be used as a raw material for the production of ferric phosphate, it can not only reduce the raw material cost of ferric phosphate, but also have an important impact on improving the overall economic efficiency of titanium dioxide production.

[0004] However, whether using ferrous sulfate (a byproduct of titanium dioxide production) or iron (a byproduct of titanium slag) as the iron source to prepare ferric phosphate, a large amount of ferric phosphate wastewater is generated during the production process. This wastewater contains approximately 10% phosphoric acid and impurities with a total metal ion content exceeding 1-ppm. Direct discharge of this wastewater not only wastes phosphoric acid resources but also pollutes the ecological environment. Therefore, to comprehensively treat and recycle ferric phosphate wastewater, the mainstream process route for wastewater treatment is currently "pretreatment + multi-stage membrane concentration + evaporation crystallization." However, this wastewater treatment process is mainly for the preparation of ferric phosphate using high-purity iron blocks as raw materials, where the wastewater is primarily phosphorus-containing with a low content of metal impurities, and the treated water can be reused as phosphoric acid. However, the wastewater generated from the production of ferric phosphate using titanium slag as a byproduct in the titanium dioxide industry contains high concentrations of phosphoric acid and metal impurity ions. Using a "pretreatment + multi-stage membrane concentration + evaporation crystallization" method for wastewater treatment only allows for the reuse of a small portion of the phosphoric acid and clean water, resulting in significant resource waste. Furthermore, the incomplete removal of metal ions after pretreatment before they enter the downstream membrane system will cause membrane element fouling and scaling, leading to irreversible damage and potentially paralyzing the entire ferric phosphate wastewater treatment system over time. Therefore, a treatment process is needed to efficiently remove metal ions, ultimately achieving phosphoric acid recovery and the reuse of crystalline salts. To address the high impurity content in ferric phosphate wastewater, industry researchers have conducted extensive studies, ultimately focusing on membrane treatment—specifically, nanofiltration membrane treatment.

[0005] Patent CN219449506U discloses a highly efficient system for removing metal ions from ferric phosphate wastewater. This method includes the following steps: dual removal of metal ions from the wastewater is achieved using ammonia dosing and an ion exchanger (chelating resin) for adsorption. This ensures that metal ions do not enter the downstream membrane concentration system. Simultaneously, the regenerated wastewater generated by the resin is not discharged outside the system to avoid environmental pollution, but is returned to the mother liquor equalization tank (the front end of the system) and re-enters the mother liquor mixing reaction tank for efficient metal ion removal. The combination of ammonia dosing and chelating resin significantly reduces the amount of ammonia required, effectively lowering the overall system's reagent costs. Ultimately, the recycled water TDS is ≤10 mg / L, and the concentrated solution TDS is ≥250,000 mg / L. However, this method pre-treats the wastewater by adding ammonia and chelating resin. The resin is expensive, resulting in high treatment costs. Furthermore, this method does not recover the phosphoric acid, leading to phosphoric acid loss and further increasing production costs.

[0006] Chinese patent application CN202010477670.8 discloses a method for recovering and treating waste phosphoric acid. By sequentially performing nanofiltration, neutralization precipitation, and electrolysis on the waste phosphoric acid, impurities in the waste phosphoric acid are separated, achieving enrichment and recovery of phosphoric acid with a high recovery rate. The recovered phosphoric acid has high purity and excellent performance, meeting the requirements of industries such as semiconductors. This application proposes nanofiltration treatment of waste phosphoric acid to obtain a nanofiltration permeate that can be directly reused for phosphoric acid, and a nanofiltration retentate that requires further removal of metal impurity ions. Although the application points out that nanofiltration can be used to treat phosphoric acid wastewater to separate metal impurity ions and phosphoric acid, the nanofiltration treatment in this application targets waste phosphoric acid, whose main component is still phosphoric acid, while the types of metal impurity ions are relatively few, such as Al mentioned in its specification. 3+ Ca 2+ or Mg 2+ Any one or at least two of the above. For situations where the object of treatment has a large number and complex types of impurity metal ions, such as wastewater from the preparation of iron phosphate cathode materials byproducts of titanium slag, the metal impurity ions are complex and difficult to remove using the methods mentioned in this application. At the same time, the presence of many magnetic impurities will also affect the capacity retention rate and charge-discharge performance of the product.

[0007] Chinese patent application CN202321664088.8 discloses a system for recovering phosphoric acid from ferric phosphate production wastewater, including a raw water tank, an ultrafiltration unit, a first-stage nanofiltration unit, a first-stage reverse osmosis unit, a second-stage nanofiltration unit, and a second-stage reverse osmosis unit. This system uses two-stage nanofiltration + reverse osmosis to purify and concentrate ferric phosphate production wastewater. Nanofiltration removes metal cations, serving as pretreatment for reverse osmosis, which can slow the accumulation of metal ions on the reverse osmosis surface and extend the lifespan of the reverse osmosis system. However, this application uses multiple (at least five) separation methods to effectively remove metal ions from the ferric phosphate production wastewater and recover the phosphoric acid, increasing consumable costs and reducing recovery efficiency. In other words, this application simply utilizes multiple separation methods without indicating how to effectively separate metal impurity ions from the ferric phosphate production wastewater using fewer separation methods, while simultaneously reusing the phosphoric acid more efficiently.

[0008] Patent CN116462347A provides a method for treating wastewater from ferric phosphate production. The method includes the following steps: (1) adjusting the pH of the wastewater to 2.2-2.5, and then separating the solid and liquid to obtain a first filtrate and a first solid; (2) adjusting the pH of the first filtrate to 5.5-6, and then separating the solid and liquid to obtain a second filtrate and a second solid; (3) adding a silicon-fluorine adsorbent and a flocculant to the second mother liquor, and then separating the solid and liquid to obtain a third filtrate and a third solid; (4) treating the third filtrate with a nanofiltration membrane to obtain a first concentrate and a first product water; (5) adjusting the pH of the first concentrate to 9-9.5, adding phosphate, and then separating the solid and liquid to obtain a fourth filtrate and a fourth solid; (6) treating the first product water and the fourth filtrate with a reverse osmosis membrane to obtain a second concentrate and a second product water; the second concentrate is rich in ammonium sulfate. This method achieves liquid-solid separation through multiple pH adjustments during wastewater treatment. However, in actual production, it requires the addition of a large amount of pH adjuster, as well as silicon-fluorine adsorbents and flocculants, resulting in high costs and the introduction of new impurities.

[0009] In summary, although the above patents all point out the separation and removal of impurities from iron phosphate production wastewater and the recycling of phosphoric acid, for cases with a large number of metal impurity ions and a specific intention to add the recycled liquid to the iron dissolving process in the lithium iron phosphate cathode material preparation process, none of the above applications indicate how to specifically and efficiently remove impurities and reuse the wastewater. Using multi-step separation methods to treat iron phosphate wastewater can easily introduce new impurities and reduce production efficiency. Summary of the Invention

[0010] Because the pig iron produced from titanium slag byproducts in the titanium industry contains 97-98% elemental iron, 1-2% carbon, and a small amount of impurities, its impurity content is higher than that of high-purity pig iron (Fe content > 99%). Consequently, the wastewater generated during the preparation of lithium iron phosphate also has a correspondingly high impurity content. Therefore, considering that the wastewater from lithium iron phosphate production, especially that from pig iron produced from titanium slag byproducts, contains a large number and complex types of metallic impurity ions, making it unsuitable for direct reuse in the production of lithium iron phosphate cathode materials, the primary objective of this invention is to control the impurity status of the two concentrates obtained in the process, thereby achieving a better impurity removal effect in the wastewater from lithium iron phosphate production.

[0011] The second objective of this invention is to control the concentration of phosphoric acid and the content of metal impurity ions in the recycled liquid obtained during the wastewater treatment process, so that it can be reused in the iron dissolving process during the preparation of lithium iron phosphate cathode materials.

[0012] The technical solution adopted by this invention to solve its technical problem is:

[0013] A method for treating wastewater from ferric phosphate production includes the following steps:

[0014] S1: Evaporation: The production wastewater is sent to an evaporation unit for concentration. After concentration, the resulting condensate is treated by a reverse osmosis membrane. The filtered water enters a reuse tank, while the remaining concentrated water is returned to the evaporation unit for further evaporation. The phosphoric acid concentration in the purified water is 0.8%–1.4%, and the total metal impurity ion content is 80–200 ppm. The remaining liquid in the evaporation unit is the bottom liquid. The phosphoric acid concentration in the production wastewater is 4%–7%, and the total metal impurity ion content is 1500–2500 ppm.

[0015] S2: First Reuse: The above-mentioned bottom solution is subjected to ICP testing to detect the metal impurity ion content, and divided into a first reuse solution and a first concentrate. The first reuse solution contains 9%–17% phosphoric acid by mass, and the total metal impurity ion content is <5000 ppm, preferably 300–450 ppm, and is sent to the iron melting process. The first concentrate includes: Ca... 2+ 390~570ppm, Co 2 + 80-140 ppm, Cr 2+ 220-280ppm, Mn 2+ 345~690ppm, Ti 2+ 15-60 ppm, Fe 3+ 3400~4100ppm.

[0016] S3: Second Reuse: The first concentrated water is subjected to nanofiltration to obtain a second reuse solution and a second concentrated water. The second reuse solution is sent to the iron dissolving process. The mass concentration of phosphoric acid in the second reuse solution is 14% to 45%, preferably 14% to 25%, and the total content of metal impurity ions is <800 ppm, preferably <400 ppm, including: Ca 2+ 10-14 ppm, Co 2+ 1-6 ppm, Cr 2+ 0-5ppm, Mn 2+ 3-27 ppm, Ti 2+ 0.6–2 ppm, Fe 3+ 80–180 ppm; the second concentrate mentioned above includes: Ca 2+ 1450~1550ppm, Co 2+ 250-350 ppm, Cr 2+ 700-800ppm, Mn 2+ 1100~1300ppm, Ti 2+ 50-170 ppm, Fe 3+ : 1200~1300ppm.

[0017] S4: Neutralization: The above-mentioned second concentrated water is neutralized sequentially with sodium hydroxide or calcium hydroxide, the amount of which is 6-13% of the mass concentration of the second concentrated water, and then separated and crystallized to obtain the by-product phosphate.

[0018] Furthermore, the aforementioned evaporation device is a triple-effect falling film evaporator, which consists of three evaporators connected in series. Low-temperature heating steam is introduced into the first effect to heat the waste liquid therein. The resulting steam is then introduced into the second effect as heating steam, causing the waste liquid in the second effect to evaporate at a lower temperature than that in the first effect. This process is repeated until the last effect. The condensate from the first effect is returned to the heat source, while the condensate from the other effects is collected and output as desalinated water. One unit of steam input can produce many times more water. Therefore, using a triple-effect falling film evaporator to evaporate and concentrate the aforementioned production wastewater can concentrate excess water while retaining more metal ions, resulting in higher treatment efficiency and lower membrane treatment costs for subsequent reverse osmosis membrane treatment.

[0019] Furthermore, the nanofiltration membrane used in the above-mentioned nanofiltration process is an NF membrane. NF membranes offer advantages such as low operating pressure, high water flux, and low operating costs. Using this type of nanofiltration membrane can better remove metal impurity ions and simultaneously recover dilute phosphoric acid from wastewater. This achieves better separation of metal impurity ions while enabling phosphoric acid reuse, thus reducing the production cost of iron phosphate. The operating pressure of nanofiltration membranes is generally below 1 MPa, resulting in low power consumption during the separation process, which is beneficial for reducing equipment investment and operating costs.

[0020] From the perspective of impurity removal in ferric phosphate wastewater, the production process generates a large amount of wastewater, which is often rich in metallic impurity ions such as Ca, Cr, Mn, and Co. These ions or combinations of ions cannot be directly discharged, and the presence of magnetic impurities can affect the capacity retention and charge / discharge performance of the product. Traditional wastewater treatment methods involving "pretreatment + multi-stage membrane concentration + evaporation crystallization" are usually ineffective in recovering the phosphoric acid solution from the wastewater. Furthermore, the large volume of wastewater and the high cost of nanofiltration membranes ultimately increase the production cost of ferric phosphate.

[0021] In the S1 evaporation and concentration stage of this application, the production wastewater is fed into an evaporation device for evaporation and concentration. The resulting condensate is then treated by a reverse osmosis membrane to obtain purified water with a phosphoric acid concentration of 0.8%–1.4% and a total metal impurity ion content of 100–200 ppm, which is then fed into a reuse tank. The condensate obtained after wastewater evaporation and concentration is then treated by a reverse osmosis membrane to remove metal cations, ultimately yielding purified water. Due to the low content of impurity ions, this purified water can partially replace pure water in the preparation of dilute phosphoric acid in the iron dissolving process.

[0022] In this step, the bottom liquid remaining in the evaporation unit after circulation evaporation is fed into the next wastewater treatment process, including nanofiltration treatment of the first concentrated water in the bottom liquid that does not meet the conditions for reuse. Because the wastewater contains not only excessively high levels of metal ions such as Ca, Cr, Mn, and Co, but also a large amount of dilute phosphoric acid, it is difficult to remove effectively using conventional methods. Furthermore, the high concentration of these ions also affects the capacity retention and charge / discharge performance of battery materials. If the iron phosphate wastewater is not treated by nanofiltration, the metal impurity ion content will be ≥5000ppm, and direct reuse in the iron dissolving process will lead to excessive impurities in the iron phosphate product, ultimately affecting the product's performance. This application selects nanofiltration treatment for this concentrated water. One characteristic of nanofiltration membranes is their ion selectivity, which can effectively remove cations from wastewater. The rejection rate for divalent and multivalent ions is significantly higher than that for monovalent ions, and it has a good treatment effect on metal ions such as Ca, Cr, Mn, and Co that are difficult to remove by conventional methods.

[0023] Then, the first recycled liquid that meets the recycling conditions in the recycled bottom liquid is subject to the process requirements of the iron dissolving process. If the metal impurity ions in the recycled water exceed the standard, it will eventually lead to the excessive impurities in the iron phosphate product, affecting the electrochemical performance of the product. The recycled liquid in this application has reached the recycling standard, and the total content of metal impurity ions has been reduced to a very low level. The impact can be ignored, and the recycling effect is good.

[0024] Similarly, in S3, nanofiltration is used for secondary treatment of wastewater. One characteristic of nanofiltration membranes is their ion selectivity; their relative molecular weight cutoff is between that of reverse osmosis and ultrafiltration membranes, and they have a certain removal rate for inorganic salts. The nanofiltration process involves raw water flowing from one end of the membrane to the other. Water molecules permeate the membrane surface, moving from the raw water side to the other, while inorganic salt ions remain on the original side. As the flow rate of the raw water gradually increases, water molecules are continuously removed, and the salt content remaining in the raw water gradually increases, meaning the raw water is gradually concentrated, eventually becoming concentrated water, which is discharged from the device. The retention effect varies for ions of different valence states; the retention rate for divalent and multivalent ions is significantly higher than that for monovalent ions. Using nanofiltration to treat the first concentrated water can achieve the effect of removing metal impurity ions while simultaneously reusing dilute phosphoric acid.

[0025] If the wastewater from ferric phosphate is not treated by nanofiltration and has a metal impurity ion content of ≥5000ppm, it will be directly reused in the iron smelting process, which will lead to excessive impurities in the ferric phosphate product and ultimately affect the product performance. If it is not reused in the iron smelting process and the wastewater is directly put into the wastewater tank, it will cause phosphoric acid pollution and waste, resulting in an increase in the production cost of ferric phosphate.

[0026] The resulting second concentrated water is then neutralized with alkali in step S4. This second concentrated water contains: Ca... 2+ 1450~1550ppm, Co 2+250-350 ppm, Cr 2+ 700-800ppm, Mn 2+ 1100~1300ppm, Ti 2+ 50-170 ppm, Fe 3 + The concentration is 1200-1300 ppm. Due to the high content of Ca, Cr, Mn and Fe ions, it is difficult to directly separate and crystallize them. Calcium hydroxide or potassium hydroxide is used for alkali neutralization. Phosphoric acid will react with calcium or sodium to generate phosphate. The main advantage is that tricalcium phosphate has good purity and high recovery efficiency, which enables the recovery and utilization of phosphorus in phosphorus-containing wastewater and achieves zero wastewater discharge.

[0027] Compared with the prior art, the embodiments of the present invention have at least the following beneficial effects:

[0028] 1. This invention uses a process of "evaporation concentration + membrane treatment + nanofiltration + neutralization" for wastewater treatment. For the small amount of concentrated liquid with excessive impurity metals generated after "evaporation concentration + membrane treatment", the metal impurity ions that are difficult to remove can be effectively removed by controlling the reuse conditions and treatment methods, so as to realize the recycling and reuse of phosphoric acid and make it suitable for the production process of iron phosphate.

[0029] 2. The concentrated water produced after nanofiltration treatment of the present invention contains phosphoric acid with a mass concentration of 8-12%. Phosphate can be obtained by alkali neutralization. It is a high-purity by-product, which can reduce production costs and effectively avoid the generation of hazardous waste, thereby reducing environmental pollution.

[0030] 3. The wastewater treatment process of this invention does not introduce new impurities, reduces the amount of reagents used, and reduces phosphoric acid loss, further optimizing the treatment cost and production cost of ferric phosphate wastewater. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments and comparative examples will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0034] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to specific embodiments.

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0036] Example 1

[0037] like Figure 1 As shown, Figure 1 This is a process flow diagram of the present invention.

[0038] S1. 500L of wastewater (phosphoric acid concentration of 4% and total metal impurity ion content of 1500ppm) generated during the production of iron phosphate using titanium slag by-products as the iron source is sent to an evaporation unit for evaporation and concentration. The resulting condensate is treated by a reverse osmosis membrane, and the filtered water enters the recycling tank. The remaining concentrated water is returned to the evaporation unit for further evaporation. The phosphoric acid concentration in the purified water is 0.8%, and the total metal impurity ion content is 80ppm. The remaining liquid in the evaporation unit is the bottom liquid.

[0039] S2. In the above-mentioned base liquid, the first reuse liquid with a phosphoric acid mass concentration of 9% and a total metal impurity ion content of 300ppm is sent to the iron dissolving process to prepare a phosphoric acid with a mass concentration of 30% for use in the pig iron dissolving process; the first concentrated water that does not meet the reuse conditions has a phosphoric acid concentration of 10% and a metal ion content of approximately 5250ppm. The specific details of the metal impurity ions are shown in Table 2.

[0040] S3. The first concentrated water is sent to the nanofiltration membrane treatment process to obtain the second reuse solution (phosphoric acid mass concentration of 14% and metal ion content of about 190 ppm) and the second concentrated water (phosphoric acid mass concentration of 9.0% and metal ion content of about 5100 ppm). The second reuse solution is sent to the iron dissolving process to prepare phosphoric acid with a mass concentration of 30%, which is used to dissolve pig iron to prepare ferrous dihydrogen phosphate solution.

[0041] S4. Neutralize the second concentrated water that does not meet the reuse conditions with Ca(OH)2 with a mass concentration of 7% to obtain a neutralized slurry. Then filter it with a filter press and dry the resulting filter cake at 130°C to obtain tricalcium phosphate as a by-product.

[0042] The removal rate of metal ions and the recovery rate of phosphoric acid in the wastewater treatment were calculated separately. The metal ion removal results are shown in Tables 1 and 2, and the phosphoric acid recovery results are shown in Tables 3 and 4. The tricalcium phosphate content of the obtained wastewater was detected, and the results are shown in Table 5.

[0043] Example 2

[0044] like Figure 1 As shown, Figure 1 This is a process flow diagram of the present invention.

[0045] S1. 500L of wastewater (phosphoric acid concentration of 5% and total metal impurity ion content of 2000ppm) generated during the production of iron phosphate using titanium slag by-products as the iron source is sent to an evaporation unit for evaporation and concentration. The resulting condensate is treated by a reverse osmosis membrane, and the filtered water enters the recycling tank. The remaining concentrated water is returned to the evaporation unit for further evaporation. The phosphoric acid concentration in the purified water is 1.0%, and the total metal impurity ion content is 300ppm. The remaining liquid in the evaporation unit is the bottom liquid.

[0046] S2. In the above-mentioned base liquid, the first reuse liquid with a phosphoric acid mass concentration of 10% and a total metal impurity ion content of 300ppm is sent to the iron dissolving process to prepare a phosphoric acid with a mass concentration of 30% for use in the pig iron dissolving process; the first concentrated water that does not meet the reuse conditions has a phosphoric acid concentration of 15% and a metal ion content of approximately 5400ppm. The specific details of the metal impurity ions are shown in Table 2.

[0047] S3. The first concentrated water is sent to the nanofiltration membrane treatment process to obtain the second reuse solution (phosphoric acid mass concentration of 16% and metal ion content of about 200 ppm) and the second concentrated water (phosphoric acid mass concentration of 11% and metal ion content of about 5200 ppm). The second reuse solution is sent to the iron dissolving process to prepare phosphoric acid with a mass concentration of 30%, which is used to dissolve pig iron to prepare ferrous dihydrogen phosphate solution.

[0048] S4. Neutralize the second concentrated water that does not meet the reuse conditions with Ca(OH)2 with a mass concentration of 7% to obtain a neutralized slurry. Then filter it with a filter press and dry the resulting filter cake at 130°C to obtain tricalcium phosphate as a by-product.

[0049] The removal rate of metal ions and the recovery rate of phosphoric acid in the wastewater treatment were calculated separately. The metal ion removal results are shown in Tables 1 and 2, and the phosphoric acid recovery results are shown in Tables 3 and 4. The tricalcium phosphate content of the obtained wastewater was detected, and the results are shown in Table 5.

[0050] Example 3

[0051] like Figure 1 As shown, Figure 1 This is a process flow diagram of the present invention.

[0052] S1. 500L of wastewater (phosphoric acid concentration of 6% and total metal impurity ion content of 2500ppm) generated during the production of iron phosphate using titanium slag by-products as the iron source is sent to an evaporation unit for evaporation and concentration. The resulting condensate is treated by a reverse osmosis membrane, and the filtered water enters the recycling tank. The remaining concentrated water is returned to the evaporation unit for further evaporation. The phosphoric acid concentration in the purified water is 1.2%, and the total metal impurity ion content is 160ppm. The remaining liquid in the evaporation unit is the bottom liquid.

[0053] S2. In the above-mentioned base liquid, the first reuse liquid with a phosphoric acid mass concentration of 12.5% ​​and a total metal impurity ion content of 330ppm is sent to the iron dissolving process to prepare a phosphoric acid mass concentration of 30% for use in the pig iron dissolving process; the first concentrated water that does not meet the reuse conditions has a phosphoric acid concentration of 15% and a metal ion content of approximately 5700ppm. The specific details of the metal impurity ions are shown in Table 2.

[0054] S3. The first concentrated water is sent to the nanofiltration membrane treatment process to obtain the second reuse solution (phosphoric acid mass concentration of 21% and metal ion content of about 240 ppm) and the second concentrated water (phosphoric acid mass concentration of 10% and metal ion content of about 5460 ppm). The second reuse solution is sent to the iron dissolving process to prepare phosphoric acid with a mass concentration of 30%, which is used to dissolve pig iron to prepare ferrous dihydrogen phosphate solution.

[0055] S4. Neutralize the second concentrated water that does not meet the reuse conditions with Ca(OH)2 with a mass concentration of 7% to obtain a neutralized slurry. Then filter it with a filter press and dry the resulting filter cake at 130°C to obtain tricalcium phosphate as a by-product.

[0056] The removal rate of metal ions and the recovery rate of phosphoric acid in the wastewater treatment were calculated separately. The metal ion removal results are shown in Tables 1 and 2, and the phosphoric acid recovery results are shown in Tables 3 and 4. The tricalcium phosphate content of the obtained wastewater was detected, and the results are shown in Table 5.

[0057] Example 4

[0058] like Figure 1 As shown, Figure 1 This is a process flow diagram of the present invention.

[0059] S1. 500L of wastewater (7% phosphoric acid concentration and 2500ppm total metal impurity ion content) generated during the production of iron phosphate using titanium slag by-products as the iron source is sent to an evaporation unit for evaporation and concentration. The resulting condensate is treated by a reverse osmosis membrane, and the filtered water enters a recycling tank. The remaining concentrated water is returned to the evaporation unit for further evaporation. The phosphoric acid concentration in the purified water is 1.4%, and the total metal impurity ion content is 200ppm. The remaining liquid in the evaporation unit is the bottom liquid.

[0060] S2. In the above-mentioned base liquid, the first reuse liquid with a phosphoric acid mass concentration of 17% and a total metal impurity ion content of 450ppm is sent to the iron dissolving process to prepare a phosphoric acid with a mass concentration of 30% for use in the pig iron dissolving process; the first concentrated water that does not meet the reuse conditions has a phosphoric acid concentration of 19% and a metal ion content of approximately 5900ppm. The specific details of the metal impurity ions are shown in Table 2.

[0061] S3. The first concentrated water is sent to the nanofiltration membrane treatment process to obtain the second reuse solution (phosphoric acid mass concentration of 24% and metal ion content of about 300 ppm) and the second concentrated water (phosphoric acid mass concentration of 11% and metal ion content of about 5600 ppm). The second reuse solution is sent to the iron dissolving process to prepare phosphoric acid with a mass concentration of 30%, which is used to dissolve pig iron to prepare ferrous dihydrogen phosphate solution.

[0062] S4. Neutralize the second concentrated water that does not meet the reuse conditions with Ca(OH)2 with a mass concentration of 7% to obtain a neutralized slurry. Then filter it with a filter press and dry the resulting filter cake at 130°C to obtain tricalcium phosphate as a by-product.

[0063] The removal rate of metal ions and the recovery rate of phosphoric acid in the wastewater treatment were calculated separately. The metal ion removal results are shown in Tables 1 and 2, and the phosphoric acid recovery results are shown in Tables 3 and 4. The tricalcium phosphate content of the obtained wastewater was detected, and the results are shown in Table 5.

[0064] Comparative Example

[0065] like Figure 1 As shown, Figure 1 This is a process flow diagram of the present invention.

[0066] S1. 500L of wastewater (phosphoric acid concentration of 4%) generated during the production of ferric phosphate using titanium slag by-product iron as the iron source is sent to an evaporation unit for evaporation and concentration. The resulting condensate is treated by a reverse osmosis membrane, and the filtered water enters a recycling tank. The remaining concentrated water is returned to the evaporation unit for further evaporation. The phosphoric acid concentration in the purified water is 0.9%, and the total amount of metal impurity ions is 140ppm. The remaining liquid in the evaporation unit is the bottom liquid.

[0067] S2. In the above-mentioned base liquid, the first reuse liquid with a phosphoric acid mass concentration of 12.5% ​​and a total metal impurity ion content of 340ppm is sent to the iron dissolving process to prepare a phosphoric acid with a mass concentration of 30% for use in the pig iron dissolving process; the first concentrated water that does not meet the reuse conditions has a phosphoric acid concentration of 15% and a metal ion content of approximately 5600ppm.

[0068] S3. Neutralize the first concentrated water with Ca(OH)2 with a mass concentration of 7% to obtain a neutralized slurry, then filter it with a filter press, and dry the obtained filter cake at 130°C to obtain tricalcium phosphate as a by-product.

[0069] The removal rate of metal ions and the recovery rate of phosphoric acid in the wastewater treatment were calculated separately. The metal ion removal results are shown in Tables 1 and 2, and the phosphoric acid recovery results are shown in Tables 3 and 4. The tricalcium phosphate content of the obtained wastewater was detected, and the results are shown in Table 5.

[0070] The implementation details of Examples 1-4 and the comparative examples are summarized in the table below:

[0071] Table 1. Metal impurity ions after 1500L wastewater treatment

[0072]

[0073] The metal ion content before and after nanofiltration treatment was measured to examine the retention rate of metal ions by the nanofiltration membrane. Since the iron phosphate wastewater mainly contains high levels of impurity ions such as calcium, cobalt, chromium, manganese, titanium, and iron, this study primarily investigated the removal of these metal ions.

[0074] Table 2. Removal of metal impurity ions from evaporation and concentration wastewater after nanofiltration treatment.

[0075]

[0076] The following table shows the phosphoric acid levels before and after treatment of the ferric phosphate wastewater in Examples 1-4 and the comparative example:

[0077] Table 3. Phosphoric acid recovery after 3500L wastewater treatment

[0078]

[0079] In Examples 1-4 and the Comparative Example, the phosphoric acid content before and after nanofiltration treatment was measured, and the phosphoric acid recovery rate was investigated, as shown in the table below:

[0080] Table 4. Phosphoric acid recovery from concentrated wastewater after nanofiltration treatment.

[0081] Example Phosphoric acid recovery rate (%) Example 1 80.29％ Example 2 79.67％ Example 3 77.54％ Example 4 75.33％ Comparative Example Without nanofiltration, the phosphoric acid recovery rate was 0%.

[0082] The test results of tricalcium phosphate obtained by separation and crystallization in Examples 1-4 and the comparative example are shown in the table below:

[0083] Table 5. Results of Tricalcium Phosphate By-product Detection in Examples

[0084] Example purity Ca% P％ Example 1 91.48％ 33.72％ 7.81 Example 2 91.65％ 34.68 8.37 Example 3 91.43％ 34.53 8.45 Example 4 91.26％ 34.37 8.71 Comparative Example 91.52％ 34.41 9.24

[0085] In summary, the simple wastewater treatment method described in this application can achieve the removal of impurity metal ions and the recovery of phosphoric acid. After nanofiltration treatment, the removal rate of metal impurity ions reaches over 90%, and the recovery rate of phosphoric acid in the wastewater reaches over 75%. Furthermore, the treated recycled water has a low content of metal impurity ions, meeting the requirements for the reuse of ferric phosphate wastewater. Compared with the comparative example, the recovery rates of phosphoric acid and water are higher, and the phosphoric acid loss is lower. The wastewater that does not meet the reuse conditions after nanofiltration treatment can be used to produce tricalcium phosphate with a purity of over 91% and a Ca content of approximately 34%, basically meeting the requirements for commercial sale.

[0086] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for treating iron phosphate production wastewater, characterized by, The method comprises the following steps: S1: evaporation: the production wastewater is sent to an evaporation device for evaporation concentration, after evaporation concentration, the liquid remaining in the evaporation device is a bottom liquid, the mass concentration of phosphoric acid in the production wastewater is 4%-7%, and the total content of metal impurity ions is 1500-2500 ppm; S2: first reuse: the bottom liquid is detected by ICP to determine the content of metal impurity ions, and is divided into a first reuse liquid and a first concentrated water, wherein the mass concentration of phosphoric acid in the first reuse liquid is 9% to 17%, the total content of metal impurity ions is 300 to 450 ppm, and the first reuse liquid is sent to a dissolving iron process; the first concentrated water includes: Ca 2+ : 390 to 570 ppm, Co 2+ : 80 to 140 ppm, Cr 2+ : 220 to 280 ppm, Mn 2+ : 345 to 690 ppm, Ti 2+ : 15 to 60 ppm, Fe 3+ : 3400 to 4100 ppm; S3: second reuse: the first concentrated water is subjected to nanofiltration treatment to obtain second reuse liquid and second concentrated water, the second reuse liquid is sent to the iron dissolution process, the mass concentration of phosphoric acid in the second reuse liquid is 14% to 45%, the total content of metal impurity ions is <800 ppm, including: Ca 2+ : 10-14 ppm, Co 2+ : 1-6 ppm, Cr 2+ : 0-5 ppm, Mn 2+ : 3-27 ppm, Ti 2+ : 0.6-2 ppm, Fe 3+ : 80-180 ppm; in the second concentrated water, including: Ca 2+ : 1450-1550 ppm, Co 2+ : 250-350 ppm, Cr 2+ : 700-800 ppm, Mn 2+ : 1100-1300 ppm, Ti 2+ : 50-170 ppm, Fe 3+ : 1200-1300 ppm; S4: neutralization: the second concentrated water is sequentially subjected to alkali neutralization and separation crystallization to prepare a by-product phosphate.

2. The method of claim 1, wherein the method is characterized by, In the S1, the condensed water obtained after the production wastewater is evaporated and concentrated is treated by a reverse osmosis membrane, the filtered clean water enters a reuse pool, and the remaining concentrated water returns to the evaporation device for continuous evaporation, the mass concentration of phosphoric acid in the clean water is 0.8%-1.4%, and the total content of metal impurity ions is 80-200 ppm.

3. The method of claim 1, wherein the method is characterized by, The mass concentration of phosphoric acid in the second reuse liquid is 14%-25%, and the total content of metal impurity ions is <400 ppm.

4. The method of claim 1, wherein the method is characterized by, In the S4, the alkali used for neutralization is sodium hydroxide or calcium hydroxide, and the amount used is 6-13% of the mass concentration of the second concentrated water.

5. The method of claim 1, wherein the method is characterized by, The evaporation device is a three-effect falling film evaporator.

6. The method of claim 1, wherein the method is characterized by, The nanofiltration membrane used in the nanofiltration is an NF membrane.

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