Wastewater recycling method for preparing iron phosphate based on raffinate acid as raw material

Abstract

The invention relates to the technical field of iron phosphate production, and discloses a wastewater recycling method for preparing iron phosphate based on raffinate acid as a raw material, which comprises the following steps: 1) dissolving ferrous sulfate heptahydrate and removing impurities to obtain a ferrous sulfate solution A; 2) removing impurities from the raffinate acid and ammonia water to obtain a phosphorus salt solution B, mixing the phosphorus salt solution B with an oxidizing agent to obtain a mixed solution C, and recycling phosphorus salt filter cake washing water for diluting the raffinate acid; (3) reacting the mixed solution C with the solution A, and performing primary filtering and washing on slurry to obtain a synthetic filter cake; and 4) mixing the synthesized filter cake, adding phosphoric acid, heating and aging, and carrying out secondary washing, drying and calcining on the aged filter cake to obtain the battery-grade iron phosphate. According to the invention, through multi-stage water circulation, phosphorus salt filter cake washing water, primary / secondary washing water graded and recycled according to conductivity and regenerated mother liquor are recycled for raw material dilution and dissolution and filter cake washing and size mixing, so that closed-loop wastewater circulation is formed, raffinate acid is used for replacing a traditional phosphorus source, and high-efficiency recovery of water resources and phosphorus resources is realized while the cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of ferric phosphate production technology, and in particular to a method for recycling wastewater from the preparation of ferric phosphate using residual raffinate as a raw material. Background Technology

[0002] As a core precursor for lithium iron phosphate batteries, the demand for iron phosphate has continued to surge, driven by the new energy vehicle and energy storage industries. However, the traditional ammonium process relies on monoammonium phosphate as the phosphorus source, leading to increased raw material costs and decreased profits. The wastewater generated in existing iron phosphate production processes contains high concentrations of phosphates and sulfates, requiring energy-intensive technologies such as multi-effect evaporation and membrane separation to meet effluent standards, resulting in high energy consumption for wastewater treatment. Furthermore, the high impurity content in raw materials during iron phosphate production and the use of high-purity water throughout the entire process lead to resource waste.

[0003] Current production technologies cannot effectively solve all of the above problems simultaneously, resulting in a failure to effectively reduce the production cost of ferric phosphate. Using residual raffinate as a raw material can significantly reduce the raw material cost of ferric phosphate production, and some companies have already implemented this production technology. However, using residual raffinate as a raw material introduces more impurities, makes wastewater treatment and recycling more difficult, and results in a lower phosphorus recovery rate.

[0004] The complex elemental composition and high content of residual acid significantly increase the complexity of wastewater treatment in the ferric phosphate production system. Based on this characteristic, the technical standards for wastewater treatment need further improvement, thus requiring the construction of a more efficient wastewater reuse and treatment system to address the problems caused by the inherent properties of the raw materials. Meanwhile, the low phosphorus yield during residual acid treatment hinders the full realization of its cost advantages. To address this issue, effective phosphorus recovery and utilization require a reasonable washing process, which also depends on a scientifically sound wastewater recycling mechanism. Furthermore, the wastewater generated from residual acid treatment contains various heavy metal ions and other pollutants; improper treatment will pose a serious threat to the environment, further highlighting the urgency and necessity of constructing a systematic and scientific wastewater treatment and reuse system. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a wastewater recycling method for preparing ferric phosphate based on residual leaching acid. Through a multi-stage water recycling system, waste acid is recycled, washing water is reused in a closed loop, and low-energy treatment is achieved. This not only significantly reduces costs but also enables efficient resource recovery and low emissions.

[0006] The solution of the present invention is: A method for recycling wastewater from the preparation of ferric phosphate using residual raffinate as a raw material, characterized by comprising the following steps: 1) Ferrous sulfate heptahydrate, ammonia, and dissolved water are mixed and subjected to a purification reaction, followed by filtration to obtain ferrous sulfate solution A; 2) After mixing the residual raffinate, ammonia, and dilution water, a purification reaction is carried out, followed by filtration to obtain phosphate solution B and phosphate filter cake. After mixing phosphate solution B with an oxidant, a mixture C is obtained. The phosphate filter cake is washed, and the resulting wash water is recycled as dilution water for the residual raffinate. 3) The mixture C and ferrous sulfate solution A are subjected to a synthesis reaction. The resulting slurry is filtered to obtain a synthesis filter cake and a primary mother liquor. The synthesis filter cake is then washed to produce primary wash water. 4) After the synthetic filter cake is mixed with slurry water for slurry preparation, phosphoric acid is added and the mixture is heated for aging. After aging, the slurry is filtered to obtain aged filter cake and secondary mother liquor. The aged filter cake is subjected to secondary washing, drying, calcination and crushing to obtain battery-grade iron phosphate. The aged filter cake is subjected to secondary washing to produce secondary wash water. The primary wash water generated from washing the synthetic filter cake and the secondary wash water generated from washing the aged filter cake are reused in stages according to their conductivity. The primary mother liquor and / or secondary mother liquor, after impurity removal treatment, are recycled as at least one of the following: dissolving water for ferrous sulfate heptahydrate, washing water for phosphate filter cake, washing water for synthetic filter cake, and slurry preparation water for synthetic filter cake.

[0007] As a preferred technical solution, when performing step 2) for the first time, the dilution water is pure water.

[0008] As a preferred technical solution, the washing water of the phosphate filter cake is recycled as dilution water, and its amount is 30% to 50% of the mass of the residual acid.

[0009] As a preferred technical solution, the primary washing water is reused as follows: when its conductivity is ≥10ms / cm, it is treated together with the primary mother liquor for impurity removal; when its conductivity is <10ms / cm, it is used as washing water for the phosphate filter cake before the next synthesis reaction.

[0010] As a preferred technical solution, the secondary washing water is reused as follows: when its conductivity is ≥3ms / cm, it is used as washing water for the filter cake at the front end of the next aging reaction; when its conductivity is <3ms / cm, it is used as filter cake conditioning water before aging.

[0011] As a preferred technical solution, the primary mother liquor and / or primary wash water with a conductivity ≥10ms / cm are mixed with a purification agent and the pH value is adjusted to 8-12 before being filtered to obtain the recycled water.

[0012] As a preferred technical solution, the secondary mother liquor is mixed with a purification agent and the pH value is adjusted to 8-12 before being reused, and then filtered to obtain the recycled water.

[0013] As a preferred technical solution, the impurity removal agent is any one of magnesium oxide, aluminum chloride, sodium carbonate, or calcium hydroxide.

[0014] Compared with the prior art, the advantages of the present invention are: 1) The wastewater recycling method for preparing ferric phosphate based on residual raffinate provided in this application fully leverages the advantages of multiple water recycling processes. It utilizes the recovered wash water and treated water from each stage as raw materials for different processes, significantly reducing the demand for fresh water, minimizing new water input, and saving water resource costs. This method is suitable for large-scale industrial continuous production in water-scarce areas. It also reduces wastewater discharge, lowers the operation and maintenance costs of wastewater treatment facilities, and greatly reduces wastewater treatment costs. This invention has significant advantages in environmental protection and efficient resource utilization, meeting the requirements of current industrial green transformation and sustainable development.

[0015] 2) The wastewater recycling method for preparing ferric phosphate based on residual raffinate provided in this application involves diluting the residual raffinate and adding it to phosphorus-containing wash water for recycling. In the residual raffinate treatment stage, the acid concentration is reduced by adding water to dilute it, thereby making it easier for subsequent impurities to be neutralized or precipitated by ammonia water. After removing impurities by adding ammonia water, the resulting filtrate is used as a subsequent phosphorus source. A large amount of water-soluble phosphorus remains in the filter cake, which is extracted by washing. The wash water is reused to dilute the residual raffinate, forming a closed-loop water recycling system. This method reduces water consumption and emissions while recovering effective phosphorus sources.

[0016] 3) The wastewater recycling method for preparing ferric phosphate based on residual raffinate provided in this application involves recycling the filter cake washing water from the synthesis and aging stages. The synthesis slurry undergoes primary filtration and washing to remove impurities. The filter cake is then aging after conditioning, followed by secondary filtration and washing. Primary plate-and-frame washing water is utilized according to its conductivity: primary washing water with a conductivity ≥10 mS / cm is treated together with the mother liquor, while primary washing water with a conductivity <10 mS / cm is used as pre-synthesis washing water for the next reaction. Secondary plate-and-frame washing water is also utilized in a tiered manner: secondary washing water with a conductivity ≥3 mS / cm is used as filter cake conditioning water before aging, while secondary washing water with a conductivity <3 mS / cm is used as pre-aging washing water for the next reaction. This wastewater recycling method can reuse water resources while avoiding excessive impurities, achieving water resource reuse while ensuring product performance and low impurity content requirements.

[0017] 4) The wastewater recycling method for preparing ferric phosphate based on residual raffinate provided in this application involves the regeneration and reuse of the mother liquor. After treatment with a purification agent, the mother liquor and wash water are primarily treated to remove ammonium sulfate, adjusting the pH and impurity levels of the filtrate. This filtrate can then be used as wash water for phosphorus-containing filter residue from the residual raffinate and for dissolving ferrous sulfate heptahydrate, thus filling the water gap at the raw material end and maintaining water balance. Simultaneously, this recycled water can be used for the lower half of the primary plate and frame filter press washing process. The filter cake, mainly composed of calcium sulfate, can be used as a building material. This achieves full recycling and utilization of materials. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the water cycle in the wastewater recycling method for preparing ferric phosphate based on residual raffinate as raw material in Example 1 of the present invention. Figure 2 This is a schematic diagram of the water cycle in the wastewater recycling method for preparing ferric phosphate based on residual raffinate as raw material in Example 1 of the present invention. Figure 3 This is a schematic diagram of the water cycle in the wastewater recycling method for preparing ferric phosphate based on residual raffinate as raw material in Example 1 of the present invention. Figure 4 This is a flowchart illustrating the equipment process for the wastewater recycling method in the preparation of ferric phosphate based on residual raffinate as a raw material, as described in Example 1 of this invention. Detailed Implementation

[0020] 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.

[0021] Example 1 See Figures 1-3The first batch (first production): Ferrous sulfate heptahydrate was filtered to remove impurities, resulting in ferrous sulfate solution A. Residual raffinate, ammonia, and pure water were continuously introduced into the phosphate salt removal tank for impurity removal. The reaction solution was filtered to obtain phosphate salt solution B, which was mixed with an oxidant to obtain mixed solution C, which was stored in a mixed solution storage tank. The phosphate salt filter cake was washed with pure water, and the phosphate salt wash water was stored. Solution A and mixed solution C were added to the synthesis reactor, and ammonia was introduced to carry out the reaction. After filtration and washing with primary pure water, a synthesis filter cake was obtained. The synthesis filter cake, slurry preparation water, and phosphoric acid were introduced into the aging reactor to complete the dispersion, heating, and aging of the slurry. The aged slurry was filtered to obtain an aged filter cake. After washing with secondary water, the aged filter cake was dried, calcined, and crushed to obtain battery-grade iron phosphate.

[0022] When the conductivity of the wash water after washing the synthetic filter cake is ≥10ms / cm, the primary wash water is sent to the mother liquor tank for subsequent processing; when the conductivity of the wash water after washing the synthetic filter cake is <10ms / cm, the primary wash water is used as the wash water for the phosphate filter cake before the next synthesis reaction.

[0023] Secondary wash water with a conductivity ≥3ms / cm after washing the aged filter cake is used as the pre-aging wash water for the next reaction, while secondary wash water with a conductivity <3ms / cm after washing the aged filter cake is used as the filter cake conditioning water before the second set of aging.

[0024] The collected primary and secondary mother liquors were adjusted to pH 10 with calcium hydroxide, filtered, and the filtrate was used as recycled water for dissolving ferrous sulfate heptahydrate in the second group and for mid-stage washing of the primary filter cake.

[0025] The second group (subsequent batches): During the treatment of residual raffinate, the wash water generated from washing the phosphate filter cake in the first group of production is used directly to dilute the residual raffinate in this group, replacing the pure water.

[0026] The synthetic filter cake is washed using a staged washing method: The initial washing stage uses recycled water that has undergone impurity removal treatment.

[0027] The middle and final washing stages use synthetic wash water with a conductivity of <10 mS / cm generated in the first group and pure water in sequence for protective washing to ensure the purity of the filter cake.

[0028] After the synthetic filter cake is washed to a qualified standard, it is slurry prepared using aging wash water with a conductivity of <3 mS / cm generated in the first group, and then subjected to aging reaction.

[0029] The washing water for the aged filter cake comes from: the aging wash water with a conductivity ≥3ms / cm generated in the first group, and pure water.

[0030] The phosphate filter cake wash water generated during the production process of this group is collected and stored for use in diluting the residual acid in the next batch of production.

[0031] The primary and secondary mother liquors and primary wash water with a conductivity ≥10 mS / cm generated in this group are collected and treated together to remove impurities. The resulting recycled water is mainly used for: dissolving the next batch of ferrous sulfate heptahydrate; and for the initial washing of the next batch of synthetic filter cake.

[0032] For any other unspecified operations, follow the procedures outlined in Group 1.

[0033] The apparatus used in this method is as follows Figure 4 As shown, ferrous sulfate heptahydrate is mixed with dissolving water (pure water in the initial reaction) and then dissolved in the ferrous sulfate dissolving tank. The solution then enters the ferrous sulfate impurity removal tank and reacts with ammonia water to remove impurities. The resulting reaction solution is filtered by the first plate and frame filter press to obtain ferrous sulfate solution A. Ferrous sulfate solution A is then buffered in the ferrous sulfate finished product tank.

[0034] Residual raffinate, ammonia, and pure water are continuously fed into the phosphate removal tank for impurity removal. The reaction solution is then filtered through a second plate and frame filter press to obtain phosphate solution B and a phosphate filter cake. Phosphate solution B is buffered in a finished phosphate tank, which is connected to a mixed solution storage tank. After entering the mixed solution storage tank, phosphate solution B is mixed with an oxidant (hydrogen peroxide) to obtain mixed solution C, which is also stored in the mixed solution storage tank. The wash water generated during the washing of the phosphate filter cake produced by the second plate and frame filter press is reused as dilution water in the phosphate removal tank. The amount of wash water used for the phosphate filter cake is 40% of the mass of the residual raffinate.

[0035] The phosphate filter cake is washed with pure water to obtain phosphate wash water, which is then stored. Mixture C and ferrous sulfate solution A are introduced into the synthesis reactor through pipelines, and then ammonia water is introduced to carry out the reaction. The synthesis reactor is connected to the pipeline of the third plate and frame filter press. After the reaction slurry is filtered, the filter cake is washed with primary pure water to obtain the synthesis filter cake. The synthesis filter cake is sent to the aging reactor and mixed with pure water for slurry preparation and dispersion. Phosphoric acid is added and the temperature is raised for aging. After aging, the slurry is sent to the fourth plate and frame filter press for filtration to obtain the aged filter cake. After drying, calcination and crushing, battery-grade iron phosphate is obtained.

[0036] The synthetic filter cake obtained from the third plate and frame filter press is washed with water that is then reused according to the different electrical conductivity values. The aging filter cake produced by the fourth plate and frame filter press is washed with water that is then reused based on its conductivity.

[0037] After the primary and secondary mother liquors are collected and an impurity removal agent is added, they are filtered and impurities are removed by the fifth plate and frame filter press to obtain recycled mother liquor and calcium sulfate filter cake. The recycled mother liquor is reused in the synthetic filter cake washing of the third plate and frame filter press, the ferrous sulfate heptahydrate dissolution water treatment, and the phosphate filter cake washing of the second plate and frame filter press.

[0038] Example 2: pH adjustment of mother liquor treatment The remaining unspecified operations shall be performed in accordance with Example 1.

[0039] The collected mother liquor and some of the wash water were mixed with calcium hydroxide to adjust the pH of the reaction system to 8.

[0040] Example 3: Sodium carbonate The remaining unspecified operations shall be performed in accordance with Example 1.

[0041] The collected mother liquor and some of the wash water were mixed with sodium carbonate to adjust the pH of the reaction system to 10.

[0042] Example 4: Adjustment of primary and secondary wash water grading and utilization nodes The remaining unspecified operations shall be performed in accordance with Example 1.

[0043] Wash water with a conductivity ≥20 mS / cm after washing the synthetic filter cake is treated together with the mother liquor. Wash water with a conductivity <20 mS / cm is used as pre-synthesis wash water for the next reaction. Wash water with a conductivity ≥8 mS / cm after washing the aged filter cake is used for preparing the synthetic filter cake slurry. Wash water with a conductivity <8 mS / cm is used as pre-aging wash water for the next reaction.

[0044] Comparative Example 1: The remaining unspecified operations shall be performed in accordance with Example 1.

[0045] Pure water was used throughout the entire process, as shown in the first set of data from Example 1.

[0046] The ferric phosphate products obtained in Examples 1-4 and Comparative Example 1 were subjected to the following tests. The test methods were performed according to commonly used methods in the art and will not be described in detail here. The results are shown in Table 1: Table 1 Results of ferric phosphate product indicators obtained in Examples 1-4 and Comparative Example 1 The product data above are all ferric phosphate product indicators obtained from the second group in the examples. Comparative Example 1 uses pure water throughout the entire process.

[0047] Examples 1-4 compared the effects of mother liquor treatment methods and wash water grading nodes on the impurity content of ferric phosphate products, and compared them with the mainstream ferric phosphate product indicators in the market. It can be seen that using the wastewater recycling process provided in this application, the impurity content of the obtained ferric phosphate product is lower than or close to the mainstream requirements. Although the impurity content is higher than that of Comparative Example 1, this method does not require the use of large amounts of pure water throughout the process; the amount of pure water used is only 50% of that in Comparative Example 1.

[0048] Data from Examples 2-4 show that the selection of the mother liquor treatment pH, the type of impurity removal agent, and the conductivity grading node of the wash water affects the impurity content of the final product. Those skilled in the art can make adaptive adjustments within the parameter range disclosed in this invention according to specific requirements for product purity.

[0049] Example 5: The difference from Example 1 is that the amount of washing water used for the phosphate filter cake is 30% of the mass of the residual acid. Magnesium oxide is added to the primary and secondary mother liquors to adjust the pH of the reaction system to 8.

[0050] Example 6: The difference from Example 1 is that the amount of washing water used for the phosphate filter cake is 50% of the mass of the residual acid. Aluminum chloride is added to the primary and secondary mother liquors to adjust the pH of the reaction system to 10.

[0051] 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 from the preparation of ferric phosphate using residual raffinate as a raw material, characterized in that, Includes the following steps: 1) Ferrous sulfate heptahydrate, ammonia, and dissolved water are mixed and subjected to a purification reaction, followed by filtration to obtain ferrous sulfate solution A; 2) After mixing the residual raffinate, ammonia, and dilution water, a purification reaction is carried out, followed by filtration to obtain phosphate solution B and phosphate filter cake. After mixing phosphate solution B with an oxidant, a mixture C is obtained. The phosphate filter cake is washed, and the resulting wash water is recycled as dilution water for the residual raffinate. 3) The mixture C and ferrous sulfate solution A are subjected to a synthesis reaction. The resulting slurry is filtered to obtain a synthesis filter cake and a primary mother liquor. The synthesis filter cake is then washed to produce primary wash water. 4) After the synthetic filter cake is mixed with slurry water for slurry preparation, phosphoric acid is added and the mixture is heated for aging. After aging, the slurry is filtered to obtain aged filter cake and secondary mother liquor. The aged filter cake is then subjected to secondary washing, drying, calcination and crushing to obtain battery-grade iron phosphate. The aged filter cake undergoes a two-stage washing process to produce secondary wash water; The primary wash water generated from washing the synthetic filter cake and the secondary wash water generated from washing the aged filter cake are reused in stages according to their conductivity. The primary mother liquor and / or secondary mother liquor, after impurity removal treatment, are recycled as at least one of the following: dissolving water for ferrous sulfate heptahydrate, washing water for phosphate filter cake, washing water for synthetic filter cake, and slurry preparation water for synthetic filter cake.

2. The wastewater recycling method for preparing ferric phosphate based on residual raffinate as a raw material, as described in claim 1, is characterized in that: When performing step 2) for the first time, the dilution water is pure water.

3. The wastewater recycling method for preparing ferric phosphate based on residual raffinate as a raw material, as described in claim 1, is characterized in that: The washing water of the phosphate filter cake, which is recycled as dilution water, is used in an amount of 30% to 50% of the mass of the residual acid.

4. The wastewater recycling method for preparing ferric phosphate based on residual raffinate as a raw material, as described in claim 1, is characterized in that... The primary wash water is reused as follows: when its conductivity is ≥10 ms / cm, it is treated together with the primary mother liquor for impurity removal; when its conductivity is <10 ms / cm, it is used as wash water for the phosphate filter cake before the next synthesis reaction.

5. The wastewater recycling method for preparing ferric phosphate based on residual raffinate as a raw material, as described in claim 1, is characterized in that... The secondary wash water is reused as follows: when its conductivity is ≥3ms / cm, it is used as the wash water for the filter cake at the front end of the next aging reaction; when its conductivity is <3ms / cm, it is used as the filter cake conditioning water before aging.

6. The wastewater recycling method for preparing ferric phosphate based on residual raffinate as a raw material, as described in claim 4, is characterized in that: Before reuse, the primary mother liquor and / or primary wash water with a conductivity ≥10 mS / cm are mixed with a purification agent and the pH value is adjusted to 8-12 before filtration to obtain the reused water.

7. The wastewater recycling method for preparing ferric phosphate based on residual raffinate as a raw material, as described in claim 5, is characterized in that: Before reuse, the secondary mother liquor is mixed with a purifying agent and the pH value is adjusted to 8-12 before filtration to obtain the recycled water.

8. A method for recycling wastewater from the preparation of ferric phosphate based on residual raffinate as a raw material, as described in claim 6 or 7, characterized in that: The impurity remover is any one of magnesium oxide, aluminum chloride, sodium carbonate, or calcium hydroxide.

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