Wastewater recycling methods and wastewater recycling systems in ferric phosphate production processes

Abstract

This invention discloses a wastewater recycling method and system for the production of ferric phosphate, comprising the following steps: ferrous sulfate impurity removal, oxidation synthesis, primary filtration and washing, filter cake re-slurrying, aging and crystallization, secondary filtration and washing, drying, calcination, crushing, demagnetization, and packaging. When the conductivity of the secondary filtration and washing water is <3000 μs / cm, the washing water is reused for filter cake re-slurrying after primary filtration and washing and for secondary filtration and washing. This invention reuses the secondary filtration and washing water according to its water quality composition characteristics for filter cake re-slurrying after primary filtration and washing, effectively utilizing valuable elements in the washing water. Simultaneously, it reduces the wastewater discharge per ton of ferric phosphate from the current 40-80 tons to 25-65 tons, reducing the load on the greywater treatment plant and lowering the production cost of ferric phosphate.

Description

Technical Field

[0001] This invention relates to the field of ferric phosphate production technology, and in particular to a wastewater recycling method and wastewater recycling system in the ferric phosphate production process. Background Technology

[0002] With increasingly stringent requirements for impurity content in lithium-ion battery precursors, most manufacturers now use the two-step ammonium iron phosphate process as the mainstream technology for producing iron phosphate. The ammonium iron phosphate two-step process uses ferrous sulfate, monoammonium phosphate, and phosphoric acid as main raw materials, employing a process flow of "ferrous sulfate impurity removal pretreatment—oxidative synthesis reaction—first filtration and washing—high-temperature crystallization aging—second filtration and washing—flash drying—high-temperature calcination—mechanical crushing—product packaging" to produce battery-grade iron phosphate. Compared to the one-step ammonium process, by adding a filtration and washing step during the reaction, impurities introduced by the ferrous sulfate raw material are separated from the solid iron phosphate from the mother liquor before high-temperature crystallization aging. This avoids the coating and encapsulation of impurities during high-temperature crystallization aging, reduces the water consumption in subsequent washing, and results in lower impurity content in the iron phosphate product. Iron phosphate products synthesized using the two-step process have advantages such as stable product quality, low impurity content, and excellent electrical performance.

[0003] The synthesis of ferric phosphate using either the ammonium or sodium process generates a large amount of wastewater. This wastewater primarily originates from two sources: one is the high-concentration ammonium sulfate or sodium sulfate solution produced during the liquid-phase precipitation process, commonly referred to as mother liquor; the other is the aqueous solution containing small amounts of phosphate and sulfate, commonly referred to as wash water, generated during the washing of the ferric phosphate precipitate to remove impurities. Specifically, the two-step ammonium process for ferric phosphate synthesis generates 10-15 tons of mother liquor and 40-80 tons of wash water per ton of ferric phosphate produced. The ratio of wash water to mother liquor is approximately 4-8 times, resulting in a total wastewater volume of 50-90 tons. This large volume of wastewater, if not properly treated, can cause severe pollution and ecological damage to the surrounding environment, significantly hindering the development of enterprises.

[0004] Currently, the industry typically treats this type of wastewater using a combination of lime precipitation, magnesium salt precipitation, membrane systems, and multi-effect evaporation. Lime precipitation suffers from persistent sludge buildup; magnesium salt precipitation involves a long process flow, consumes large amounts of magnesium salts and gases, resulting in high processing costs and difficulty in achieving compliant discharge standards. Therefore, most manufacturers opt for a combination of membrane systems and multi-effect evaporation to treat ferric phosphate wastewater. The treated pure water is reused in the system, while the byproducts, ammonium sulfate and ammonium phosphate, are sold as products. However, this method also has drawbacks such as large wastewater treatment volume, complex and long process flow, large footprint of the wastewater treatment system, and high investment costs.

[0005] To reduce wastewater treatment costs, existing technologies describe methods for recycling ferric phosphate wash water. For example, Chinese patent CN116374976A discloses a method for recycling ferric phosphate production and its application. However, this technology simply returns the secondary wash water to the first washing step after precipitation. The quality of the wash water generated in the secondary washing process is not constant; the conductivity of the wash water varies at different washing endpoints, ranging from a maximum of 10–20 μS / cm to a minimum of 200 μS / cm. In contrast, existing technologies only involve a single washing step... The conductivity of the wash water needs to be reduced to ≤2.0 mS / m, and the conductivity of the wash water for the second wash needs to be reduced to ≤500 μS / cm. If all the second wash water is used for the first wash, the wash water will not only be wasted to some extent because it is not classified according to the change of impurity content over time, but also ineffective washing time will be generated, affecting production efficiency. In the two-step process of producing ferric phosphate using ammonium, the existing technology still uses pure water or demineralized water for washing. For production enterprises, the water requirements and water consumption will result in high costs.

[0006] In addition, when the wash water generated in the initial stage of secondary washing has a high conductivity and a high impurity content, if this part of the wash water is used for primary washing, the impurities are likely to form water-insoluble solid precipitates and accumulate on the surface of the filter cake. During the crystallization and aging process, the impurities are secondary coated and cannot be removed by washing in the future, which ultimately leads to an increase in the impurity content of the product. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a method and system for recycling wastewater in the production process of iron phosphate.

[0008] The solution of the present invention is:

[0009] A method for recycling wastewater in the production process of ferric phosphate includes the following steps: removing impurities from ferrous sulfate, oxidative synthesis, primary filtration and washing, filter cake re-slurrying, aging and crystallization, secondary filtration and washing, drying, calcination, crushing, demagnetization and packaging.

[0010] When the conductivity of the wash water from the secondary filtration washing is less than 3000 μs / cm, the wash water is reused in the re-slurry of the filter cake after the primary filtration washing and in the secondary filtration washing water. When the conductivity of the wash water from the secondary filtration washing is between 1000 μs / cm and 3000 μs / cm, it is stored in the first water storage tank and returned for the re-slurry of the filter cake after the primary filtration washing. When the conductivity of the wash water from the secondary filtration washing is less than 1000 μs / cm, it is stored in the second water storage tank and returned for the secondary filtration washing water.

[0011] Compared to primary plate and frame washing, secondary plate and frame washing water is characterized by higher phosphorus (P) content and relatively lower impurity content. A crucial means of controlling impurity content in the ammonium process two-step process is ensuring the effectiveness of primary plate and frame washing. During primary washing, impurities should be removed as much as possible to prevent unremoved impurities from forming secondary coatings with iron phosphate crystals during crystallization and aging. Experiments have shown that these coated impurities cannot be removed by subsequent washing methods, even with increased washing water volume. Therefore, from the perspective of improving product quality and controlling the final impurity content, to reduce the risk of poor primary plate and frame washing effect and impurity residue caused by secondary washing water, the primary plate and frame washing should still use the highest quality pure water or desalinated water. Meanwhile, since the primary plate and frame washing has removed most of the metal impurities, the filter cake obtained from the secondary plate and frame washing has a much lower concentration of impurity elements on its surface compared to the filter cake obtained from the primary plate and frame washing. The purpose of the secondary plate and frame washing is mainly to remove excess phosphate, phosphate and residual sulfate from the mother liquor during aging and crystallization. The requirements for water quality are relatively low. Washing water with a conductivity of <1000μs / cm can meet the production requirements.

[0012] As a preferred technical solution, when the conductivity of the wash water in the secondary filtration and washing process is >3000μs / cm, the wash water is discharged into a drain tank for storage and then sent to a greywater treatment device for processing.

[0013] As a preferred technical solution, when the conductivity of the wash water stored in the first water storage tank is >3000μs / cm, the wash water stored in the first water storage tank is stirred evenly and then discharged into the drain tank.

[0014] As a preferred technical solution, when the conductivity of the wash water stored in the second water storage tank is >2000μs / cm, the wash water stored in the second water storage tank is stirred evenly and then discharged into the drain tank.

[0015] When the conductivity of the wash water stored in the second water storage tank is >2000 μs / cm, the wash water stored in the second water storage tank should be stirred evenly and then discharged into the drain tank. Through evaluation experiments on the reuse of wash water for secondary plate and frame filter cake washing at different washing endpoints (6000, 5000, 4000, 3000, 2000 μs / cm), it was found that when the conductivity of the wash water is higher than 2000 μs / cm, the impurity content in the wash water is high, mainly composed of P, Mg, and Mn. Using this portion of the wash water for reuse will cause secondary enrichment of impurities in the ferric phosphate filter cake, resulting in excessively long washing time and increased product impurity content. However, when using secondary plate and frame filter cake wash water with a conductivity lower than 2000 μs / cm for reuse to wash the filter cake, the water quality is better and the impurity content is lower, so it will not affect the washing effect or product quality. In addition, when the solid impurity storage tank has a liquid level mark at the bottom that exceeds 2 cm, the wash water stored in the storage tank needs to be stirred evenly and then discharged into the drain tank.

[0016] The present invention also discloses a wastewater recycling system for the iron phosphate production process, which is applicable to the wastewater recycling method in the iron phosphate production process of the present invention.

[0017] As a preferred technical solution, the system includes a plate and frame filter press, a first water storage tank, a second water storage tank, a drain tank, a mother liquor tank, and an online conductivity meter. The online conductivity meter is installed at the outlet of the plate and frame filter press. The plate and frame filter press is connected to the mother liquor tank via a mother liquor discharge pipe. The plate and frame filter press is connected to the drain tank via a first outlet pipe. The plate and frame filter press is connected to the first water storage tank via a second outlet pipe. The plate and frame filter press is connected to the second water storage tank via a third outlet pipe. The first water storage tank is connected to the re-slurry tank in the filter cake re-slurry step via a first reuse pipe. The second water storage tank is connected to the inlet pipe of the plate and frame filter press via a second reuse pipe.

[0018] As a preferred technical solution, the first water storage tank is connected to the drainage tank through a first fixed drain pipe; the second water storage tank is connected to the drainage tank through a second fixed drain pipe.

[0019] As a preferred technical solution, the mother liquor tank is connected to the drain tank through a first drain pipe; the drain tank is connected to the greywater device through a second drain pipe.

[0020] The plate and frame filter press is equipped with a two-stage filter plate and frame.

[0021] The above-mentioned technical solution provides a wastewater recycling method and system for a ferric phosphate production process, comprising the following steps: ferrous sulfate impurity removal, oxidation synthesis, primary filtration and washing, filter cake re-slurrying, aging and crystallization, secondary filtration and washing, drying, calcination, crushing, demagnetization, and packaging; when the conductivity of the secondary filtration and washing water is <3000 μs / cm, the washing water is reused in the re-slurrying of the filter cake after primary filtration and washing, and in the secondary filtration and washing water; when the conductivity of the secondary filtration and washing water is 1000 μs / cm to 3000 μs / cm, it is stored in a first storage tank and returned for the re-slurrying of the filter cake after primary filtration and washing; when the conductivity of the secondary filtration and washing water is <1000 μs / cm, it is stored in a second storage tank and returned for the secondary filtration and washing water.

[0022] Advantages of this invention:

[0023] 1) When the conductivity of the wash water is higher than 3000 μS / cm, the impurity content in the wash water is high, mainly composed of P, Mg, and Mn. Reusing this portion of the wash water can cause secondary enrichment of impurities in the ferric phosphate filter cake, resulting in excessively long washing times and increased product impurity content. However, using wash water with a conductivity lower than 3000 μS / cm for resizing the filter cake after the first filtration wash, and using water with a conductivity lower than 1000 μS / cm for resizing the second-stage plate and frame washing, results in better water quality and lower impurity content, achieving a balance between impurity content and washing time. Industrial-scale verification has shown that this does not affect the washing effect or product quality. Furthermore, because the initial wash water from the second-stage plate and frame washing has a high P content, using it as resizing water after the first filtration wash helps to evenly disperse the filter cake during resizing. Compared to using water alone for resizing, this can increase the product's tap density from 0.60–0.70 g / cm³. 3 Increased to 0.80–0.90 g / cm³ 3 This makes the primary particle size more uniform, preventing uneven and incomplete crystallization during high-temperature aging caused by agglomeration, which could lead to fluctuations in the Fe / P ratio, increased impurity content, and inconsistent primary particle size. As a result, a ferric phosphate product with consistent quality and good stability between batches was obtained.

[0024] 2) The wastewater recycling method in the iron phosphate production process provided by this invention can reduce the amount of pure water or desalinated water used in the secondary washing process. In addition, since the washing water in the secondary washing process has a certain temperature, the energy consumption required for heating can be reduced during reuse and subsequent treatment. After the washing water is reused, the wastewater discharge per ton of iron phosphate can be reduced from the current 40-80 tons to 25-65 tons, reducing the treatment load of the greywater treatment unit and reducing the production cost of iron phosphate.

[0025] 3) The wastewater recycling method in the iron phosphate production process of the present invention involves periodically discharging the wash water and sediment after the secondary wash water has been stored for a certain period of time by detecting the conductivity of the first and second water storage tanks. This reduces the impact of impurity enrichment caused by the increased number of cycles. The impurities in the wash water collected in the first water storage tank mainly consist of a small amount of FePO4 solid suspended solids that pass through the filter during the initial plate and frame washing process and PO4. 3- The main impurity ions are Mg and Mn. Since the pH of the wash water is roughly between 4 and 5, under these pH conditions, PO4... 3-When Mg and Mn are present simultaneously, solid precipitates of Mg3(PO4)2 and Mn3(PO4)2 are formed. If these precipitates accumulate on the surface of the ferric phosphate filter cake during wash water reuse, it will cause an increase in the impurity content of the product (mainly Mn and Mg). Since Mg3(PO4)2 and Mn3(PO4)2 are insoluble in water, the above impurities cannot be washed away by subsequent washing. However, by periodically discharging the precipitates and wash water in the first and second water storage tanks through conductivity testing, the accumulation of impurities caused by recycled water can be controlled. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of the wastewater recycling method in the iron phosphate production process of the present invention.

[0027] Figure 2 This is a schematic diagram of the iron phosphate wastewater recycling system of the present invention. Detailed Implementation

[0028] This invention provides a method and wastewater recycling system for the production of iron phosphate.

[0029] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0030] Example 1:

[0031] like Figure 1 As shown, a method for recycling wastewater in a ferric phosphate production process is disclosed. The ferric phosphate production process includes the following steps: ferrous sulfate impurity removal, oxidation synthesis, primary filtration and washing, filter cake re-slurrying, crystallization aging, secondary filtration and washing, drying, calcination, crushing, demagnetization, and packaging. When the conductivity of the wash water from the secondary filtration and washing is <3000 μs / cm, the wash water is reused for filter cake re-slurrying after primary filtration and washing and for secondary filtration and washing water.

[0032] When the conductivity of the secondary filtration washing water is 1000 μs / cm to 3000 μs / cm, it is sent to the first water storage tank for storage and returned for filter cake re-slurrying; when the conductivity of the secondary filtration washing water is <1000 μs / cm, it is sent to the second water storage tank for storage and returned for secondary filtration washing water.

[0033] When the conductivity of the secondary filtration washing water is >3000μs / cm, the washing water is discharged into the drain tank and sent to the greywater treatment device for treatment.

[0034] When the conductivity of the wash water stored in the first water storage tank is >3000μs / cm, the wash water stored in the first water storage tank is stirred evenly and then discharged into the drain tank.

[0035] When the conductivity of the wash water stored in the second water storage tank is >3000μs / cm, the wash water stored in the second water storage tank is stirred evenly and then discharged into the drain tank.

[0036] In this embodiment, the amount of pure water or demineralized water used per ton of ferric phosphate is reduced by 15 tons, and the wastewater discharge per ton of product is reduced from the current 75 tons to 60 tons. The impurity content in the product is shown in Table 1 below. The main impurity indicators are consistent with the mainstream ammonium method two-step process, and some indicators of the present invention are superior to the mainstream ammonium method two-step process.

[0037] Table 1. Results of Physicochemical Analysis and ICP Comparison Analysis of Products

[0038] project Mainstream ammonium process two-step process The process of this invention Calcium (Ca), ppm 24.46 19.43 Magnesium (Mg), ppm 34.45 25.17 Sodium (Na), ppm 10.54 9.43 Potassium (K), ppm 10.62 10.32 Copper (Cu), ppm 1.68 1.78 Zinc (Zn), ppm 24.41 10.65 Nickel (Ni), ppm 0.00 0.00 Manganese (Mn), ppm 48.56 30.25 Titanium (Ti), ppm 5.63 2.44 Aluminum (Al), ppm 10.28 8.18 Chromium (Cr), ppm 5.33 5.26 Cobalt (Co), ppm 1.53 1.29 Sulfur (S), ppm 80.22 55.58 pH 3.10 3.25 <![CDATA[Moisture (H2O), ppm ≤]]> 3000 2800 <![CDATA[Tap density, g / m 3 > 0.70 0.95

[0039] Example 2:

[0040] like Figure 1 and Figure 2 As shown, a wastewater recycling system in the iron phosphate production process is applicable to the wastewater recycling method in the iron phosphate production process of Example 1.

[0041] The system includes a plate and frame filter press, a first water storage tank, a second water storage tank, a drain tank, a mother liquor tank, and an online conductivity meter. The online conductivity meter is installed at the outlet of the plate and frame filter press. The plate and frame filter press is connected to the mother liquor tank via a mother liquor discharge pipe. The plate and frame filter press is connected to the drain tank via a first outlet pipe. The plate and frame filter press is connected to the first water storage tank via a second outlet pipe. The plate and frame filter press is connected to the second water storage tank via a third outlet pipe. The first water storage tank is connected to the inlet pipe of the plate and frame filter press via a first reuse pipe. The second water storage tank is connected to the re-slurry tank in the filter cake re-slurry step via a second reuse pipe.

[0042] The first water storage tank is connected to the drainage tank via a first fixed drain pipe; the second water storage tank is connected to the drainage tank via a second fixed drain pipe.

[0043] The mother liquor tank is connected to the greywater device via a first drain pipe; the drain tank is connected to the greywater device via a second drain pipe.

[0044] The plate and frame filter press is equipped with a secondary filter frame.

[0045] In this embodiment, control valves are installed on the mother liquor discharge pipe, the first water outlet pipe, the second water outlet pipe, the third water outlet pipe, the first reuse pipe, the second reuse pipe, the first fixed discharge pipe, the second fixed discharge pipe, the first drain pipe, and the second drain pipe. Conductivity is automatically detected by an online conductivity meter, and the opening and closing of the control valves is controlled by a programmed control system, thus controlling the opening and closing of each pipeline. This achieves full automation of the washing process, eliminating the need for manual operation and inspection, significantly saving labor costs and reducing the errors and delays of manual inspection, thereby improving production efficiency.

[0046] Example 3:

[0047] Ferric phosphate was prepared using the wastewater recycling system in the ferric phosphate production process of Example 2 and the wastewater recycling method in the ferric phosphate production process of Example 1. The only difference was that when the conductivity of the wash water from the secondary filtration washing was <6000 μS / cm, all the wash water was stored in the first storage tank and returned to the secondary filtration washing process itself. The amount of pure water or desalinated water used per ton of ferric phosphate was reduced by 10 tons, and the wastewater discharge per ton of product was reduced from the current 72 tons to 62 tons. The impurity content in the product is shown in Table 2 below.

[0048] Table 2. Results of Physicochemical Analysis and ICP Comparison Analysis of Products

[0049]

[0050]

[0051] As can be seen from Table 2, when the conductivity of the recycled washing water increases to <6000 μs / cm, the content of the main impurities in the product increases significantly, which is significantly worse than that of the iron phosphate product produced by the mainstream ammonium two-step process.

[0052] Example 4: Ferric phosphate was prepared using the wastewater recycling system in the ferric phosphate production process of Example 2 and the wastewater recycling method in the ferric phosphate production process of Example 1. The only difference was that when the conductivity of the wash water from the secondary filtration washing was <5000 μs / cm, all the wash water was stored in the first storage tank and returned to the secondary filtration washing itself. The amount of pure water or desalinated water used per ton of ferric phosphate was reduced by 12 tons, and the wastewater discharge per ton of product was reduced from the current 72 tons to 60 tons. The impurity content in the product is shown in Table 3 below.

[0053] Table 3. Results of Physicochemical Analysis and ICP Comparison Analysis of Products

[0054] project Mainstream ammonium process two-step process This embodiment Calcium (Ca), ppm 24.46 24.75 Magnesium (Mg), ppm 34.45 65.06 Sodium (Na), ppm 10.54 11.34 Potassium (K), ppm 10.62 12.53 Copper (Cu), ppm 1.68 1.54 Zinc (Zn), ppm 24.41 25.35 Nickel (Ni), ppm 0.00 0.00 Manganese (Mn), ppm 48.56 82.63 Titanium (Ti), ppm 5.63 5.49 Aluminum (Al), ppm 10.28 15.22 Chromium (Cr), ppm 5.33 6.84 Cobalt (Co), ppm 1.53 1.41 Sulfur (S), ppm 80.22 89.26 pH 3.10 3.18 <![CDATA[Moisture (H2O), ppm ≤]]> 3000 2800 <![CDATA[Tap density, g / m 3 > 0.70 0.81

[0055] As can be seen from Table 3, when the conductivity of the recycled washing water decreases to <5000μs / cm, the content of the main impurities in the product tends to decrease, but it is still inferior to the iron phosphate product produced by the mainstream ammonium two-step process.

[0056] Example 5:

[0057] Ferric phosphate was prepared using the wastewater recycling system in the ferric phosphate production process of Example 2 and the wastewater recycling method in the ferric phosphate production process of Example 1. The only difference was that when the conductivity of the wash water from the secondary filtration washing was <4000 μS / cm, all the wash water was stored in the first storage tank and returned to the secondary filtration washing itself. The amount of pure water or demineralized water used per ton of ferric phosphate was reduced by 13 tons, and the wastewater discharge per ton of product was reduced from the current 72 tons to 59 tons. The impurity content in the product is shown in Table 4 below.

[0058] Table 4. Results of Physicochemical Analysis and ICP Comparison Analysis of Products

[0059] project Mainstream ammonium process two-step process This embodiment Calcium (Ca), ppm 24.46 24.75 Magnesium (Mg), ppm 34.45 50.58 Sodium (Na), ppm 10.54 11.27 Potassium (K), ppm 10.62 11.29 Copper (Cu), ppm 1.68 1.74 Zinc (Zn), ppm 24.41 23.45 Nickel (Ni), ppm 0.00 0.00 Manganese (Mn), ppm 48.56 63.28 Titanium (Ti), ppm 5.63 5.71 Aluminum (Al), ppm 10.28 13.42 Chromium (Cr), ppm 5.33 6.09 Cobalt (Co), ppm 1.53 1.62 Sulfur (S), ppm 80.22 81.75 pH 3.10 3.19 <![CDATA[Moisture (H2O), ppm ≤]]> 3000 2900 <![CDATA[Tap density, g / m 3 > 0.70 0.84

[0060] As can be seen from Table 4, when the conductivity of the recycled washing water decreases to <4000μs / cm, the content of the main impurities in the product tends to decrease, but some indicators are still worse than those of the mainstream ammonium two-step process iron phosphate product.

[0061] Example 6:

[0062] Ferric phosphate was prepared using the wastewater recycling system in the ferric phosphate production process of Example 2 and the wastewater recycling method in the ferric phosphate production process of Example 1. The only difference was that when the conductivity of the wash water from the secondary filtration and washing process was <6000 μS / cm, all the wash water was stored in the second storage tank and returned to the filter cake for re-slurrying. The amount of pure water or demineralized water used per ton of ferric phosphate was reduced by 15 tons, and the wastewater discharge per ton of product was reduced from the current 72 tons to 57 tons. The impurity content in the product is shown in Table 5 below.

[0063] Table 5. Results of Physicochemical Analysis and ICP Comparison Analysis of Products

[0064] project Mainstream ammonium process two-step process This embodiment Calcium (Ca), ppm 24.46 27.58 Magnesium (Mg), ppm 34.45 75.11 Sodium (Na), ppm 10.54 12.57 Potassium (K), ppm 10.62 15.82 Copper (Cu), ppm 1.68 1.53 Zinc (Zn), ppm 24.41 28.26 Nickel (Ni), ppm 0.00 0.00 Manganese (Mn), ppm 48.56 102.74 Titanium (Ti), ppm 5.63 8.17 Aluminum (Al), ppm 10.28 17.27 Chromium (Cr), ppm 5.33 7.48 Cobalt (Co), ppm 1.53 1.77 Sulfur (S), ppm 80.22 103.53 pH 3.10 3.16 <![CDATA[Water (H2O), ppm ≤]]> 3000 2900 <![CDATA[Tap density, g / m 3 > 0.70 0.76

[0065] As can be seen from Table 5, when the conductivity of the recycled primary plate and frame filter cake washing water is increased to <6000 μs / cm, the content of the main impurities in the product increases significantly, which is significantly worse than that of the ferric phosphate product produced by the mainstream ammonium method two-step process.

[0066] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for recycling wastewater in a ferric phosphate production process, characterized in that, The process includes the following steps: ferrous sulfate purification, oxidation synthesis, primary filtration and washing, filter cake re-slurrying, aging and crystallization, secondary filtration and washing, drying, calcination, crushing, demagnetization and packaging; When the conductivity of the wash water from the secondary filtration washing is less than 3000 μs / cm, the wash water is reused for the re-slurry of the filter cake after the primary filtration washing and for the secondary filtration washing water. When the conductivity of the wash water from the secondary filtration washing is between 1000 μs / cm and 3000 μs / cm, it is stored in the first water storage tank and returned for the re-slurry of the filter cake after the primary filtration washing. When the conductivity of the wash water from the secondary filtration washing is less than 1000 μs / cm, it is stored in the second water storage tank and returned for the secondary filtration washing water.

2. The wastewater recycling method in the iron phosphate production process as described in claim 1, characterized in that: When the conductivity of the wash water during the secondary filtration and washing process is >3000 μs / cm, the wash water is discharged into a drainage tank for storage and then sent to a greywater treatment plant.

3. The wastewater recycling method in the iron phosphate production process as described in claim 1, characterized in that: When the conductivity of the wash water stored in the first water storage tank is >3000μs / cm, the wash water stored in the first water storage tank is stirred evenly and then discharged into the drain tank.

4. The wastewater recycling method in the iron phosphate production process as described in claim 1, characterized in that: When the conductivity of the wash water stored in the second water storage tank is >2000μs / cm, the wash water stored in the second water storage tank is stirred evenly and then discharged into the drain tank.

5. A wastewater recycling system for ferric phosphate production, characterized in that: The wastewater recycling system in the ferric phosphate production process is applicable to the wastewater recycling method in the ferric phosphate production process described in any one of claims 1 to 4.

6. The wastewater recycling system for ferric phosphate production as described in claim 5, characterized in that: The system includes a plate and frame filter press, a first water storage tank, a second water storage tank, a drain tank, a mother liquor tank, and an online conductivity meter. The online conductivity meter is installed at the outlet of the plate and frame filter press. The plate and frame filter press is connected to the mother liquor tank via a mother liquor discharge pipe. The plate and frame filter press is connected to the drain tank via a first outlet pipe. The plate and frame filter press is connected to the first water storage tank via a second outlet pipe. The plate and frame filter press is connected to the second water storage tank via a third outlet pipe. The first water storage tank is connected to the re-slurry tank in the filter cake re-slurry step via a first reuse pipe. The second water storage tank is connected to the inlet pipe of the plate and frame filter press via a second reuse pipe.

7. The wastewater recycling system for ferric phosphate production as described in claim 6, characterized in that: The first water storage tank is connected to the drainage tank through a first fixed drain pipe; the second water storage tank is connected to the drainage tank through a second fixed drain pipe.

8. A wastewater recycling system for ferric phosphate production as described in claim 6, characterized in that, The mother liquor tank is connected to the drainage tank via a first drainage pipe; the drainage tank is connected to the greywater device via a second drainage pipe.

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