Treatment and recovery process of ferrous oxalate synthesis wastewater

Abstract

The application discloses a ferrous oxalate wastewater treatment and recovery process, and belongs to the field of industrial wastewater treatment. The ferrous oxalate wastewater synthesized by a liquid phase precipitation method from ferrous sulfate solution and ammonium oxalate solution is used as raw material, residual iron oxalate complex in the wastewater is removed by adding an appropriate amount of potassium permanganate solution, and the solution obtained after removing the precipitated impurities is evaporated and crystallized to generate mixed ammonium sulfate / potassium sulfate crystals. The evaporated condensate water enters a subsequent conveying system into a material synthesis section and is recycled as production water. The application overcomes the problems of high iron impurity content in wastewater, ineffective recovery of secondary product ammonium sulfate, acid wastewater pollution and the like in the preparation process of the mainstream process for synthesizing ferrous oxalate. The process can effectively realize the treatment of iron impurities in ferrous oxalate wastewater and the recycling of ammonium sulfate resources, and provides a solution for realizing an economic, efficient and green production process of ferrous oxalate.

Description

Technical Field

[0001] This invention belongs to the field of inorganic chemical wastewater treatment, specifically relating to a treatment and recovery process for ferrous oxalate synthesis wastewater. Background Technology

[0002] Ferrous oxalate (FeC2O4·2H2O) is an important inorganic chemical raw material, traditionally widely used in dyes, coatings, ceramic colorants, and photosensitive materials. With the rapid development of the lithium-ion battery industry, the demand for ferrous oxalate, a key precursor for the synthesis of lithium iron phosphate (LiFePO4) cathode materials, has increased dramatically. However, as the scale of the ferrous oxalate industry continues to expand, the environmental problems caused by its production process are becoming increasingly prominent. Currently, the industrial production of ferrous oxalate mainly adopts the liquid-phase precipitation method, employing two reaction schemes: the reaction of ferrous sulfate with ammonium oxalate and the reaction of ferrous sulfate with oxalic acid, followed by post-processing steps such as centrifugation, washing, and drying to obtain the product. This process generates a large amount of process sulfuric acid or ammonium sulfate wastewater. This wastewater is characterized by high acidity, the presence of oxalate iron complexes, and organic matter; direct discharge not only wastes valuable resources but also causes serious environmental pollution. For example, Chinese patent CN201410257234.4, "A Clean Preparation Method of High-Purity Ultrafine Ferrous Oxalate," discloses a clean preparation method for ferrous oxalate. The preparation process involves first dissolving solid ferrous sulfate and oxalic acid separately in water and purifying them. Then, the aqueous solution of ferrous sulfate or oxalic acid is pretreated with dispersants such as alcohols to obtain a high-purity ultrafine ferrous oxalate product. The by-product sulfuric acid waste liquid is returned directly to dissolve elemental iron without treatment and participates in the recycling reaction.

[0003] Current technologies primarily involve sending process wastewater to the plant's wastewater treatment plant for neutralization and sedimentation before reusing it in production processes. This approach has several drawbacks: traditional liquid-phase precipitation methods result in low raw material utilization and purity, leading to limited resource recovery value; simple wastewater treatment methods struggle to effectively remove accumulated complexed iron impurities, resulting in large wastewater volumes and high purification costs. More importantly, none of these methods address the recovery of high-concentration ammonium sulfate / sulfuric acid from the wastewater, leading to resource waste. Summary of the Invention

[0004] The technical problem this invention aims to solve is the difficulties encountered in existing ferrous oxalate synthesis processes, such as large volumes of acidic wastewater, difficulty in removing impurities, and ineffective recovery of byproducts. This invention provides a highly efficient treatment and recovery process for ferrous oxalate synthesis wastewater. The objective of this invention is achieved through the following technical solution, the specific steps of which are as follows:

[0005] Step (1): Add ascorbic acid, which accounts for 3% to 5% of the mass of ferrous sulfate, to the ferrous sulfate solution, mix well, and then add ammonium oxalate solution to react. After the reaction is completed, filter to obtain ferrous oxalate crystals and wastewater containing oxalate iron complex. The Fe²⁺ concentration in the wastewater is 0.1 to 3 g / L.

[0006] Step (2): Heat the wastewater obtained in step (1) to 60℃~90℃, and add Fe... 2+ A potassium permanganate solution with a concentration of 0.1~0.5 mol / L was added dropwise at a molar ratio of 1:(1.05~1.1) to potassium permanganate. The mixture was stirred at a constant temperature for 1h~3h. After the reaction was complete, the mixture was filtered, and the primary precipitate and primary filtrate were collected. The primary precipitate was a mixture of amorphous ferric hydroxide and trace amounts of manganese dioxide.

[0007] Step (3): Take the filtrate obtained in step (2) and continue to add potassium permanganate solution. The amount of potassium permanganate solution used is 0.67 to 0.7 times the volume of potassium permanganate solution in step (2). React at a constant temperature of 60℃ to 90℃ for 1 to 3 hours. After the reaction is complete, filter and collect the secondary precipitate and the secondary filtrate. The secondary precipitate is δ-type manganese dioxide.

[0008] Step (4): The secondary filtrate obtained in step (3) is evaporated, concentrated and crystallized to obtain ammonium sulfate / potassium sulfate mixed crystals;

[0009] Step (5): The water condensed from the evaporation in step (4) is tested and qualified, and then transported to step (1) for recycling as production water.

[0010] The beneficial effects of this invention are:

[0011] This invention develops a treatment and recovery process for ferrous oxalate synthesis wastewater, maximizing the economical production and product value of ferrous oxalate materials. The wastewater generated from the synthesis of ferrous oxalate using ammonium oxalate and ferrous sulfate solutions contains impurities such as oxalate complexes, and the wastewater is generally acidic, requiring oxidation and precipitation treatment to reduce the environmental harm of byproducts. The core concept of this invention lies in: actively introducing potassium permanganate as a multifunctional reagent, and through a precisely controlled two-step oxidation reaction, sequentially achieving:

[0012] (1) Oxidation-"complex breaking"-hydrolysis precipitation of oxalate-complexed iron; In view of the high stability of iron complexes in wastewater, the present invention introduces potassium permanganate solution to destroy the structure of complexes in wastewater by utilizing its strong oxidizing properties, so that Fe 2+ Oxidized to Fe 3+ The process involves hydrolysis, polymerization, and precipitation, and the simplified schematic equation is: [Fe(C2O4)2] 2- + MnO4 - + 5H+ → Mn 2+ + 4CO2↑+ H2O + Fe(OH)3↓, to ensure complete oxidation of oxalate complexed iron in wastewater, the amount of potassium permanganate used in step (2) is based on Fe 2 + The molar ratio of Fe to potassium permanganate was controlled at 1:(1.05~1.1) to ensure that Fe 2+ Fully oxidized to Fe 3+ And hydrolyze and precipitate, excess MnO 4- With Mn generated in solution 2+ The reaction produces δ-MnO2 (2MnO 4- + 3Mn 2+ + 2H₂O → 5MnO₂↓ + 4H + ).

[0013] (2) Excess Mn 2+ With MnO4 - The "disproportionation reaction" converts the MnO2 into precipitate; the application of the "disproportionation reaction" is the key innovation of this invention: after the first oxidation reaction is completed, the waste liquid still contains MnO2 generated in the first reaction. 2+ and potentially excessive MnO4 - This invention completely removes Mn element by adding a specific amount of potassium permanganate to precipitate it as δ-MnO2 (2MnO). 4 - + 3Mn 2+ + 2H₂O → 5MnO₂↓ + 4H + This reaction not only eliminated Mn contamination, but also generated H₂. + It can partially compensate for the H consumed in the first step of the reaction. + This helps maintain pH stability during the subsequent evaporation and crystallization process, while also reducing the acidity of the solution.

[0014] Through the first two steps, the harmful Fe and introduced Mn in the wastewater are reduced to below 0.1 mg / L, while the remaining K element in the potassium permanganate is retained. The ammonium sulfate / potassium sulfate mixed crystals generated by subsequent evaporation and crystallization (with K content only around 2%) can be directly applied to the production of compound fertilizers in the fertilizer industry. The distillation-condensate water can be recycled back to the initial material synthesis process. This invention successfully achieves a highly efficient treatment process for removing harmful elements and recovering valuable elements from ferrous oxalate wastewater. The final retained K... + With the NH4 already present in the wastewater + SO4 2- Together they form a mixed system of ammonium sulfate and potassium sulfate that can be utilized as resources. Attached Figure Description

[0015] Figure 1 This is a flowchart of a high-efficiency treatment process for ferrous oxalate synthesis wastewater;

[0016] Figure 2 These are X-ray diffraction patterns of two precipitation processes during wastewater purification; the upper curve represents the primary precipitation Fe(OH)3, and the lower curve represents the secondary precipitation MnO2.

[0017] Figure 3 This is an X-ray diffraction pattern of a mixed crystal of ammonium sulfate and potassium sulfate. Table 1 shows the metal element composition analysis of the wastewater before and after iron removal; Table 2 shows the composition analysis of the ammonium sulfate / potassium sulfate crystals formed by crystallization; and Table 3 shows the pH value data of the wastewater before and after impurity removal. Detailed Implementation

[0018] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0019] like Figure 1 As shown, the efficient treatment and recycling process for this type of wastewater involves the following specific steps:

[0020] Example 1:

[0021] Step (1): Add ascorbic acid (3%~5% by mass of ferrous sulfate) to the ferrous sulfate solution, mix well, and then add it to the ammonium oxalate solution to react. After the reaction is complete, filter to obtain ferrous oxalate crystals and wastewater containing ferric oxalate complexes. The Fe in the wastewater... 2+ The concentration is 0.77 g / L;

[0022] Step (2): Heat the wastewater obtained in step (1) to 60°C, and then... 2+ A 0.2 mol / L potassium permanganate solution was added dropwise at a molar ratio of 1:1.05 to potassium permanganate. The mixture was stirred at a constant temperature for 3 hours. After the reaction was complete, the mixture was filtered, and the primary precipitate and primary filtrate were collected. The primary precipitate was a mixture of amorphous ferric hydroxide and trace amounts of manganese dioxide.

[0023] Step (3): Take the first filtrate obtained in step (2) and continue to add potassium permanganate solution. The amount of potassium permanganate solution used is 0.7 times the volume of potassium permanganate solution in step (2). React at a constant temperature of 60℃ for 3 hours. After the reaction is complete, filter and collect the secondary precipitate and the secondary filtrate. The secondary precipitate is δ-type manganese dioxide.

[0024] Step (4): The secondary filtrate obtained in step (3) is evaporated, concentrated and crystallized to obtain ammonium sulfate / potassium sulfate mixed crystals, wherein the mass percentage of potassium element is 2.159%.

[0025] Tables 1 and 2 show that the Fe content in the wastewater and the Fe content in the crystallized ammonium sulfate / potassium sulfate crystals decreased significantly before and after iron removal. The introduced manganese was also removed through a "disproportionation reaction," with both Fe and Mn contents approaching 0.001%, thus achieving the goal of iron removal. After impurity removal, the pH of the wastewater increased from 3.8 to approximately 6.8, and the acidity of the solution decreased significantly to neutral (Table 3). This is due to the oxidation reaction of the oxalate complex, which consumed a large amount of H₂. + This causes the pH value to rise.

[0026] Table 1

[0027] <![CDATA[Before Fe removal 2+ the content of metal elements in the solution]]> 769.15 - - <![CDATA[Removal of Fe by potassium permanganate 2+ Content of metal elements in the subsequent solution]]> 0.027 0.062 249.2

[0028] Table 2

[0029] <![CDATA[Unremoved Fe 2+ mixed crystals obtained by evaporative crystallization]]> 15500(1.55%) ---- ---- <![CDATA[Potassium permanganate for Fe removal 2+ followed by evaporative crystallization to obtain mixed crystals]]> 9.542（0.001%） 14.218（0.001%） 21585.3（2.159%）

[0030] Table 3

[0031] pH 3.8 6.8

[0032] Example 2:

[0033] Step (1): Add ascorbic acid (3%~5% by mass of ferrous sulfate) to the ferrous sulfate solution, mix well, and then add it to the ammonium oxalate solution to react. After the reaction is complete, filter to obtain ferrous oxalate crystals and wastewater containing ferric oxalate complexes. The Fe in the wastewater... 2+ The concentration was 2.17 g / L;

[0034] Step (2): Heat the wastewater obtained in step (1) to 75°C, and then... 2+ A 0.2 mol / L potassium permanganate solution was added dropwise at a molar ratio of 1:1.05 to potassium permanganate. The mixture was stirred at a constant temperature for 3 hours. After the reaction was complete, the mixture was filtered, and the primary precipitate and primary filtrate were collected.

[0035] Step (3): Take the first filtrate obtained in step (2) and continue to add potassium permanganate solution. The amount of potassium permanganate solution used is 0.7 times the volume of potassium permanganate solution in step (2). React at a constant temperature of 75°C for 3 hours. After the reaction is complete, filter and collect the secondary precipitate and secondary filtrate.

[0036] Step (4): The secondary filtrate obtained in step (3) is evaporated, concentrated and crystallized to obtain ammonium sulfate / potassium sulfate mixed crystals;

[0037] The X-ray diffraction analysis curves of the two precipitations obtained in this embodiment are as follows: Figure 2 As shown, the primary precipitate obtained in this embodiment is amorphous ferric hydroxide. This is because the addition of potassium permanganate solution triggers the oxidation reaction of the iron complex. The resulting ferric hydroxide exhibits no obvious characteristic diffraction peaks in XRD, as shown in the figure. Figure 2 As shown in the curve above, there may be trace amounts of MnO2, which is to ensure the removal of all Fe. 2+ Adding an excess of potassium permanganate solution causes a "disproportionation reaction" of manganese, generating some manganese dioxide. Figure 2 The curve below is the XRD curve of the secondary precipitation, which corresponds completely to the standard PDF #80-1098 δ-MnO2 card, verifying that the crystallized sample is δ-MnO2.

[0038] Figure 3 The XRD curves of the (NH4)2SO4 / K2SO4 mixed crystals after evaporation and crystallization show that the mixed crystals are mainly composed of (NH4)2SO4 crystals, which corresponds to the (NH4)2SO4 card in standard PDF #00-040-0660, while the K element content is only about 2%.

[0039] Example 3:

[0040] Step (1): Add ascorbic acid (3%~5% by mass of ferrous sulfate) to the ferrous sulfate solution, mix well, and then add it to the ammonium oxalate solution to react. After the reaction is complete, filter to obtain ferrous oxalate crystals and wastewater containing ferric oxalate complexes. The Fe in the wastewater... 2+ The concentration was 0.50 g / L, and the pH was 3.7.

[0041] Step (2): Heat the wastewater obtained in step (1) to 75°C, and then... 2+ A 0.5 mol / L potassium permanganate solution was added dropwise at a molar ratio of 1:1.05 to potassium permanganate. The mixture was stirred at a constant temperature for 1 hour. After the reaction was complete, the mixture was filtered, and the primary precipitate and primary filtrate were collected. The primary precipitate was a mixture of amorphous ferric hydroxide and trace amounts of manganese dioxide.

[0042] Step (3): Take the first filtrate obtained in step (2) and continue to add potassium permanganate solution. The amount of potassium permanganate solution used is 0.7 times the volume of potassium permanganate solution in step (2). React at a constant temperature of 75°C for 1 hour. After the reaction is complete, filter and collect the secondary precipitate and the secondary filtrate. The secondary precipitate is δ-type manganese dioxide.

[0043] Step (4): The secondary filtrate obtained in step (3) is evaporated, concentrated and crystallized to obtain ammonium sulfate / potassium sulfate mixed crystals.

[0044] Example 4:

[0045] Step (1): Add ascorbic acid (3%~5% by mass of ferrous sulfate) to the ferrous sulfate solution, mix well, and then add it to the ammonium oxalate solution to react. After the reaction is complete, filter to obtain ferrous oxalate crystals and wastewater containing ferric oxalate complexes. The Fe in the wastewater... 2+The concentration is 0.50 g / L;

[0046] Step (2): Heat the wastewater obtained in step (1) to 60°C, and then... 2+ A 0.5 mol / L potassium permanganate solution was added dropwise at a molar ratio of 1:1 to potassium permanganate. The mixture was stirred at a constant temperature for 1 hour. After the reaction was complete, the mixture was filtered, and the primary precipitate and primary filtrate were collected. The primary precipitate was a mixture of amorphous ferric hydroxide and trace amounts of manganese dioxide.

[0047] Step (3): Take the first filtrate obtained in step (2) and continue to add potassium permanganate solution. The amount of potassium permanganate solution used is 0.7 times the volume of potassium permanganate solution in step (2). React at a constant temperature of 60℃ for 1 hour. After the reaction is complete, filter and collect the secondary precipitate and the secondary filtrate. The secondary precipitate is δ-type manganese dioxide.

[0048] Step (4): The secondary filtrate obtained in step (3) is evaporated, concentrated and crystallized to obtain ammonium sulfate / potassium sulfate mixed crystals.

[0049] Comparative example:

[0050] Step (1): Add ascorbic acid (3%~5% by mass of ferrous sulfate) to the ferrous sulfate solution, mix well, and then add it to the ammonium oxalate solution to react. After the reaction is complete, filter to obtain ferrous oxalate crystals and wastewater containing ferric oxalate complexes. The Fe in the wastewater... 2+ The concentration was 2172.05 mg / L;

[0051] Step (2): Heat the wastewater obtained in step (1) to 60°C, and then... 2+ Hydrogen peroxide with a concentration of 1 mol / L was added dropwise at a molar ratio of 1:1 to hydrogen peroxide, and the mixture was stirred at a constant temperature for 3 hours. No precipitate was formed in the mixed solution.

[0052] In this embodiment, hydrogen peroxide was used as the oxidant for iron removal experiments. No ferric hydroxide precipitate appeared after thorough stirring. This indicates that the oxalate complex was not effectively destroyed, and hydrogen peroxide cannot be used as an oxidant for iron removal in ammonium oxalate and ferrous sulfate wastewater systems.

[0053] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A process for treating and recovering ferrous oxalate synthesis wastewater, the specific steps of which are as follows: Step (1): Add ascorbic acid at a mass of 3% to 5% of ferrous sulfate to the ferrous sulfate solution, mix well, and then add it to the ammonium oxalate solution to react. After the reaction is completed, filter to obtain ferrous oxalate crystals and wastewater containing oxalate iron complex. The Fe²⁺ concentration in the wastewater is 0.1 to 3 g / L. Step (2): Heat the wastewater obtained in step (1) to 60℃~90℃, and add Fe... 2+ Add potassium permanganate solution with a concentration of 0.1~0.5 mol / L dropwise at a molar ratio of 1:(1.05~1.1) to potassium permanganate, stir at a constant temperature for 1h~3h, filter after the reaction is complete, and collect the first precipitate and the first filtrate. Step (3): Take the first filtrate obtained in step (2) and continue to add potassium permanganate solution. The amount of potassium permanganate solution used is 0.67 to 0.7 times the volume of potassium permanganate solution in step (2). React at a constant temperature of 60℃ to 90℃ for 1 to 3 hours. After the reaction is complete, filter and collect the secondary precipitate and secondary filtrate. Step (4): The secondary filtrate obtained in step (3) is evaporated, concentrated and crystallized to obtain ammonium sulfate / potassium sulfate mixed crystals; Step (5): The water condensed from the evaporation in step (4) is tested and qualified, and then transported to step (1) for recycling as production water.

2. The process according to claim 1, characterized in that: In step (1), ascorbic acid is added before heating the ferrous sulfate solution to prevent Fe... 2+ Oxidation.

3. The process according to claim 1, characterized in that: The primary precipitate in step (2) is a mixture of amorphous ferric hydroxide and trace amounts of manganese dioxide.

4. The process according to claim 1, characterized in that: The secondary precipitate in step (3) is δ-type manganese dioxide.

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