A method for co-producing ferrous oxalate dihydrate and sulfuric acid

Abstract

This invention relates to the field of titanium dioxide by-product resource utilization and chemical synthesis technology, specifically to a method for co-producing ferrous oxalate dihydrate and sulfuric acid. The method includes the following steps: mixing solid FeSO4·7H2O and solid H2C2O4·2H2O, stirring and reacting at 70-80℃ for 2-4 hours to obtain a slurry; performing solid-liquid separation while hot to obtain a first filtrate and a ferrous oxalate dihydrate filter cake; washing and drying the ferrous oxalate dihydrate filter cake with hot water to obtain ferrous oxalate dihydrate; cooling and allowing the first filtrate to stand, followed by solid-liquid separation to obtain sulfuric acid and unreacted solid raw materials; concentrating the sulfuric acid through multi-effect evaporation, and obtaining concentrated sulfuric acid after solid-liquid separation. This method requires no additional water addition during the reaction process, produces a high concentration of sulfuric acid, enabling sulfuric acid recovery; and achieves a high iron recovery rate.

Description

Technical Field

[0001] This invention relates to the field of titanium dioxide by-product resource utilization and chemical synthesis technology, and more specifically, to a method for co-producing ferrous oxalate dihydrate and sulfuric acid. Background Technology

[0002] In the sulfuric acid process for titanium dioxide production, approximately 3.5 to 4 tons of ferrous sulfate heptahydrate (commonly known as ferrous sulfate sulfate) are produced as a byproduct for every ton of titanium dioxide produced. Currently, the main method for preparing ferrous oxalate from this byproduct is the total solution method. This involves dissolving ferrous sulfate in a large amount of water, adding oxalic acid as a precipitant, and adjusting the pH to a suitable range by adding alkali to promote the reaction.

[0003] However, this technical approach has significant drawbacks: the sulfuric acid produced in the reaction is diluted with a large amount of water and further neutralized by alkali, resulting in an extremely low concentration of sulfuric acid in the final filtrate (usually far below 10% by mass). Low-concentration sulfuric acid is not economically viable for direct recovery, and the cost of multi-effect evaporation concentration far exceeds the value of the sulfuric acid itself; the alkali addition operation also introduces additional salt, increasing wastewater treatment costs.

[0004] Theoretically, reducing water usage or omitting alkali could enrich the sulfuric acid produced in the system, increasing its concentration and facilitating subsequent recovery. However, the reaction of oxalic acid with ferrous sulfate (FeSO4 + H2C3O4) presents challenges. The reaction FeC₂O₄↓ + H₂SO₄ is a typical reversible reaction affected by acidity. According to Le Chatelier's principle, an increase in sulfuric acid concentration will lead to an increase in H₂ in the system. + As the concentration increases, high-concentration sulfuric acid significantly inhibits the forward displacement reaction thermodynamically, leading to a marked decrease in iron yield. Therefore, under the current technological framework, it is difficult to simultaneously achieve high iron yield and sulfuric acid recovery, ultimately resulting in almost no economic viability for waste acid recovery, forcing it to be treated as hazardous wastewater.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for co-producing ferrous sulfate dihydrate and sulfuric acid using solid ferrous sulfate heptahydrate and solid oxalic acid dihydrate as raw materials, based on a quasi-solid-phase reaction mechanism of in-situ release of water of crystallization. This method is simple to operate, requires no additional water during the reaction process, and only uses a small amount of water for product washing, thus significantly reducing the overall water consumption compared to the traditional solution method. The sulfuric acid produced by the reaction has a concentration as high as 33%~37%, which can be directly used to react with reduced ilmenite to prepare synthetic rutile. It can also be evaporated and concentrated for reuse in the acidolysis process of sulfuric acid titanium dioxide, and the iron recovery rate is high.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A method for co-producing ferrous oxalate dihydrate and sulfuric acid includes the following steps: S1. Mix solid FeSO4·7H2O and solid H2C2O4·2H2O, and stir the mixture at 70~80℃ using the crystal water released in situ from the raw materials as the reaction medium for 2~4 hours to obtain a slurry; S2. While the slurry is still hot, a first solid-liquid separation is performed to obtain a first filtrate and a ferrous oxalate dihydrate filter cake; S3. The ferrous oxalate dihydrate filter cake is washed and dried with hot water to obtain ferrous oxalate dihydrate; After cooling and allowing the first filtrate to stand, a second solid-liquid separation is performed to obtain sulfuric acid and unreacted solid raw materials.

[0008] Preferably, in step S3, after the second solid-liquid separation, the following steps are further included: multi-effect evaporation and concentration of the sulfuric acid and third solid-liquid separation to obtain concentrated sulfuric acid.

[0009] Preferably, in step S1, the FeSO4·7H2O is a byproduct generated during the sulfuric acid process for titanium dioxide production.

[0010] Preferably, in step S1, the molar ratio of FeSO4·7H2O to H2C2O4·2H2O is 1:(0.95~1.05); and / or, the stirring speed is 200~800 rpm.

[0011] Preferably, in step S2, the temperature of the first solid-liquid separation is ≥60℃.

[0012] Preferably, in step S2, the mass concentration of H2SO4 in the first filtrate is 33%~37%.

[0013] Preferably, in step S3, the temperature of the hot water is 50~60℃, and the filter cake is washed until the conductivity of the washing liquid is ≤1000µS / cm.

[0014] Preferably, in step S3, the first filtrate is cooled to 0~10℃ and then allowed to stand for 2~6 hours; and / or, the drying temperature is 60~80℃ and the drying time is 2~8 hours.

[0015] Preferably, in step S3, the unreacted solid raw material obtained from the second solid-liquid separation is returned to step S1 for recycling.

[0016] Preferably, the concentrated sulfuric acid contains H2SO4 at a mass concentration of ≥55%; and / or, the concentrated sulfuric acid contains iron at a content of ≤3000ppm.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention abandons the conventional solution method, directly mixing solid ferrous sulfate heptahydrate and solid oxalic acid dihydrate, and releasing the water of crystallization in situ at 70-80°C to form a slurry reaction system with a low liquid-to-solid ratio. A locally highly supersaturated environment is constructed at the reaction interface, driving the nucleation and precipitation of ferrous oxalic acid dihydrate through strong crystallization kinetics, effectively mitigating the inhibition of the reaction's thermodynamic equilibrium by the high-acid environment. It significantly increases the concentration of sulfuric acid in the post-reaction filtrate (up to 33%-37%) while ensuring an iron recovery rate of ≥90%. Compared to the traditional solution method, this invention significantly reduces water consumption and significantly increases the concentration of sulfuric acid generated, allowing it to be directly used in the reaction with reduced ilmenite to prepare synthetic rutile without concentration. Furthermore, it can be concentrated and reused in the acidolysis process of sulfuric acid-based titanium dioxide, achieving sulfuric acid recycling. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0019] The first aspect of the present invention provides a method for co-producing ferrous oxalate dihydrate and sulfuric acid, comprising the following steps: S1. Mix solid FeSO4·7H2O and solid H2C2O4·2H2O, and stir the mixture at 70~80℃ using the crystal water released in situ from the raw materials as the reaction medium for 2~4 hours to obtain a slurry; S2. While the slurry is still hot, perform the first solid-liquid separation to obtain the first filtrate and ferrous oxalate dihydrate filter cake; S3. The filter cake of ferrous oxalate dihydrate is washed and dried with hot water to obtain ferrous oxalate dihydrate; After cooling and allowing the first filtrate to stand, a second solid-liquid separation is performed to obtain sulfuric acid (second filtrate) and unreacted solid raw materials.

[0020] In the method of this invention, the solid raw material releases its water of crystallization in situ under temperature and stirring, serving as a local reaction medium. A displacement reaction occurs, generating ferrous oxalate dihydrate and sulfuric acid. The reaction system then transforms into a slurry of light yellow precipitate and sulfuric acid enrichment. After the reaction is completed, the slurry is subjected to a first solid-liquid separation while hot to obtain ferrous oxalate dihydrate filter cake and a first filtrate (crude acid). The ferrous oxalate dihydrate is washed with hot water to remove impurities. The final ferrous oxalate dihydrate can be used as an industrial-grade iron source intermediate, directly for high-temperature calcination to prepare high-quality iron oxide pigments, ferrite magnetic materials, and iron-based catalysts. In addition, this product can also be used as a high-grade iron-containing raw material, which, after further purification, can be used as a precursor for the cathode material of new energy batteries. The first filtrate is cooled and allowed to stand. Taking advantage of the fact that the solubility of residual oxalic acid dihydrate and ferrous sulfate heptahydrate decreases sharply at low temperatures in a high-concentration sulfuric acid system, they are forced to crystallize out. After a second solid-liquid separation, sulfuric acid and unreacted solid raw materials are obtained. The unreacted solid raw materials can be recycled. The obtained sulfuric acid can be directly used to react with reduced ilmenite to prepare synthetic rutile, which basically achieves zero discharge of waste acid.

[0021] This invention abandons the traditional solution method, directly mixing solid ferrous sulfate heptahydrate and solid oxalic acid dihydrate without adding any external liquid solvent. It relies on the water of crystallization released from the raw materials as the reaction medium to form a low liquid-to-solid ratio reaction system. A locally highly supersaturated environment is constructed at the reaction interface, and strong crystallization kinetics drive the nucleation and precipitation of ferrous oxalic acid dihydrate, effectively mitigating the inhibition of the high-acid environment on the thermodynamic equilibrium of the reaction. This invention uses only a small amount of water in the washing process, significantly reducing water consumption compared to traditional solution methods. This significantly increases the concentration of sulfuric acid in the first filtrate, enabling the recovery and reuse of sulfuric acid while ensuring an iron recovery rate of ≥90%.

[0022] In some embodiments, typically but not limitingly, for example, the temperature of the stirring reaction in step S1 can be any one value or a range of any two values ​​from 70°C, 72°C, 75°C, 78°C, and 80°C; the stirring reaction time can be any one value or a range of any two values ​​from 2h, 2.5h, 3h, 3.5h, and 4h.

[0023] In some specific embodiments of the present invention, step S3, after the second solid-liquid separation, further includes the following steps: multi-effect evaporation concentration and a third solid-liquid separation to obtain concentrated sulfuric acid. Multi-effect evaporation further increases the sulfuric acid concentration and achieves deep iron removal. As water evaporates and the sulfuric acid concentration further increases, residual ferrous oxalate crystallizes out due to the common ion effect, and high-purity concentrated sulfuric acid is obtained after the third solid-liquid separation. The method of the present invention employs a purification process of "hot filtration to separate crude acid, cooling crystallization to precipitate residual dihydrate oxalic acid and heptahydrate ferrous sulfate, and forced precipitation of residual iron salts through evaporation concentration," effectively improving the concentration and purity of the concentrated sulfuric acid. The obtained concentrated sulfuric acid can be reused in the acidolysis process of the sulfuric acid process for titanium dioxide production, essentially achieving zero discharge of waste acid. Compared with the traditional solution method, the method of the present invention significantly reduces water consumption and significantly increases the concentration of sulfuric acid generated, effectively reducing the energy consumption cost of the evaporation concentration process.

[0024] In some specific embodiments of the present invention, FeSO4·7H2O used in step S1 is a byproduct generated during the production of titanium dioxide using the sulfuric acid process; this method can realize the resource utilization of ferrous sulfate heptahydrate byproduct of titanium dioxide production using the sulfuric acid process, reducing environmental and cost pressures.

[0025] In some specific embodiments of the present invention, the molar ratio of FeSO4·7H2O and H2C2O4·2H2O in step S1 is 1:(0.95~1.05), for example, it can be any one value or a range of any two values ​​among 1:0.95, 1:0.98, 1:1, 1:1.02, and 1:1.05; and / or, the stirring speed is 200~800 rpm, for example, it can be any one value or a range of any two values ​​among 200 rpm, 400 rpm, 600 rpm, and 800 rpm.

[0026] In some specific embodiments of the present invention, the temperature of the first solid-liquid separation in step S2 is ≥60°C. For example, it can be any one value or a range of any two values ​​among 60°C, 62°C, 65°C, 68°C, and 70°C. If the temperature is too low, it may cause unreacted oxalic acid dihydrate to crystallize, causing the filter cake to clump and making filtration difficult.

[0027] In some specific embodiments of the present invention, the mass concentration of H2SO4 in the first filtrate in step S2 is 33% to 37%, for example, it can be any one value or a range of any two values ​​from 33%, 34%, 35%, 35.5%, 36%, 36.5%, 37%.

[0028] In some specific embodiments of the present invention, the temperature of the hot water used for washing the filter cake in step S3 is 50~60℃. For example, it can be any one value or a range of any two values ​​among 50℃, 52℃, 55℃, 58℃, and 60℃. If the temperature of the hot water used for washing is too low, the washing effect will be poor. If the temperature is too high, it will cause energy waste and increase costs. The filter cake is washed until the conductivity of the washing liquid is ≤1000µS / cm. The washing liquid is collected separately and is not combined with the filtrate.

[0029] In some specific embodiments of the present invention, in step S3, the first filtrate is cooled to 0-10°C and then allowed to stand. For example, the cooled temperature can be any value or a range of any two values ​​from 0°C, 2°C, 5°C, 8°C, and 10°C; the standing time is 2-6 hours, for example, any value or a range of any two values ​​from 2 hours, 3 hours, 4 hours, 5 hours, and 6 hours; and / or, the drying temperature is 60-80°C, for example, any value or a range of any two values ​​from 60°C, 65°C, 70°C, 75°C, and 80°C; the drying time is 2-8 hours, for example, any value or a range of any two values ​​from 2 hours, 4 hours, 6 hours, and 8 hours. The purpose of controlling the cooling temperature is to reduce the solubility of residual oxalic acid dihydrate and ferrous sulfate heptahydrate in the first filtrate, causing them to crystallize and precipitate.

[0030] In some specific embodiments of the present invention, in step S3, the unreacted solid raw material obtained from the second solid-liquid separation is returned to step S1 for recycling, which effectively improves the utilization rate of the raw material.

[0031] In some specific embodiments of the present invention, in step S3, the small amount of iron salt after the third solid-liquid separation can be incorporated into the raw materials for recycling.

[0032] In some specific embodiments of the present invention, the mass concentration of H2SO4 in the obtained concentrated sulfuric acid is ≥55%, for example, it can be any one value or a range of any two values ​​among 55%, 56%, 60%, 62%, and 65%; and / or, the iron content in the concentrated sulfuric acid is ≤3000ppm, for example, it can be any one value or a range of any two values ​​among 3000ppm, 2800ppm, 2600ppm, 2480ppm, 2410ppm, and 2330ppm.

[0033] The embodiments of the present invention will be described in detail below with reference to specific examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0034] Example 1 S1. 278g of FeSO4·7H2O crystals, a byproduct of the sulfuric acid process for titanium dioxide production, and 126g of industrial H2C2O4·2H2O crystals were directly added into a jacketed stirred reactor without adding any liquid solvent. Stirring was started, and the temperature was raised to 70°C using hot water in the jacket. It was observed that the solid system gradually turned into a slurry by releasing its own water of crystallization. The reaction was carried out at 70°C and stirred at 400 rpm for 2.5 h to obtain the slurry. S2. After the reaction is complete, while still hot (≥60℃), discharge the slurry obtained in step S1 into a centrifuge for solid-liquid separation to obtain a wet filter cake (ferrous oxalate dihydrate filter cake) and the first filtrate; S3. The wet filter cake was washed with hot water at 60℃ until the conductivity at the washing endpoint was 985 µS / cm, and then dried at 70℃ for 7h to obtain ferrous oxalate dihydrate product; the washing liquid was collected separately and not combined with the first filtrate. The first filtrate was cooled to 8°C and allowed to stand for 5 hours to precipitate crystals. Solid-liquid separation was performed, and 30.12g of crystals were separated and returned to step S1 for recycling. The obtained sulfuric acid (second filtrate) was concentrated by multi-effect evaporation and filtered to obtain concentrated sulfuric acid. The small amount of solid after filtration was added to the raw materials for reuse.

[0035] The purity of ferrous oxalate dihydrate in the product obtained in this embodiment is 98.3%. The mass fraction of H2SO4 in the first filtrate is 35.61%, the mass fraction of H2SO4 in the obtained concentrated sulfuric acid is 55.8%, and the total Fe content in the concentrated sulfuric acid is 2480 ppm. The iron recovery rate of this process is 90.4%.

[0036] Example 2 Example 2 is similar to Example 1, except that in step S1, 139g of FeSO4·7H2O crystals (byproduct of the sulfuric acid process for titanium dioxide) and 63g of industrial H2C2O4·2H2O are directly added to a jacketed stirred reactor without adding any liquid solvent. The temperature is raised to 70°C using hot water in the jacket, and the reaction is stirred for 2 hours. In step S3, the first filtrate is cooled to 5°C and allowed to stand for 5 hours, resulting in the precipitation of 14.57g of crystals. All other process conditions are the same as in Example 1.

[0037] In this embodiment, the purity of ferrous oxalate dihydrate is 98.8%. The mass fraction of H2SO4 in the first filtrate is 33.37%, the mass fraction of H2SO4 in the obtained concentrated sulfuric acid is 56.04%, and the total Fe content in the concentrated sulfuric acid is 2330 ppm. The iron recovery rate of this process is 90.8%.

[0038] Example 3 Example 3 is similar to Example 1, except that in step S1, 139g of FeSO4·7H2O crystals, a byproduct of the sulfuric acid process for titanium dioxide, and 63g of industrial H2C2O4·2H2O are directly added into a jacketed stirred reactor without adding any liquid solvent. The temperature is raised to 80°C by using hot water in the jacket, and the reaction is stirred for 4 hours. All other process conditions are the same as in Example 1.

[0039] In this embodiment, the purity of ferrous oxalate dihydrate is 98.9%. The mass fraction of H2SO4 in the first filtrate is 36.28%, the mass fraction of H2SO4 in the obtained concentrated sulfuric acid is 55.9%, and the total Fe content in the concentrated sulfuric acid is 2410 ppm. The iron recovery rate of this process is 90.5%.

[0040] Comparative Example 1 79.9 g of FeSO4·7H2O, a byproduct of the sulfuric acid process for titanium dioxide, was dissolved in 100 mL of water. 36.21 g of industrial H2C2O4·2H2O was dissolved in 90 g of water. The oxalic acid solution was slowly added to the ferrous sulfate heptahydrate solution. The reaction temperature was 70℃, the rotation speed was 400 rpm, and the pH of the solution was measured to be 0.55 after 2.5 h. The solution was then filtered, washed with deionized water at 60℃, and the conductivity at the washing endpoint was 982 µS / cm. The product was dried at 70℃ for 7 h to obtain ferrous oxalate dihydrate. The purity of ferrous oxalate dihydrate was 99.1%.

[0041] The mass fraction of H2SO4 in the main filtrate was found to be only 10.87%, the concentration of by-product sulfuric acid was low and had no economic recovery value; and the iron recovery rate of this process was only 78.9%.

[0042] Comparative Example 2 79.9 g of FeSO4·7H2O (a byproduct of the sulfuric acid process for titanium dioxide) was dissolved in 100 mL of water, and 36.21 g of industrial H2C2O4·2H2O was dissolved in 100 g of water. The oxalic acid solution was slowly added to the ferrous sulfate heptahydrate solution, while simultaneously adding 10 wt% NaOH solution dropwise to control the pH of the reaction system to 3-4. The reaction was carried out at 70℃ with stirring at 400 rpm for 2.5 h. The mixture was then filtered, washed with deionized water at 60℃ (the final conductivity was 975 µS / cm), and dried at 70℃ for 7 h to obtain the ferrous oxalate dihydrate product. The purity of the ferrous oxalate dihydrate product was 99.2%, showing no significant improvement in product quality compared to Comparative Example 1.

[0043] The pH of the main filtrate was measured to be 3.6, indicating that the generated sulfuric acid had been largely neutralized to Na₂SO₄ by the alkali solution, resulting in almost complete loss of the byproduct sulfuric acid. Furthermore, due to the low Na₂SO₄ concentration in the tail liquid, recovery was not economically viable, and treating it as wastewater increased wastewater treatment costs. While the iron recovery rate of this process was 99.6%, compared to the embodiments of this invention, it came at the cost of sacrificing the full recovery value of the byproduct sulfuric acid and incurring additional wastewater treatment burdens, with limited improvement in product quality (purity increase of less than 1%).

[0044] In Comparative Example 1, no sodium hydroxide solution was added to the reaction system. Although the concentration of sulfuric acid generated was somewhat higher than that in Comparative Example 2, it was still very low, making direct recovery difficult. Evaporation and concentration costs were high, resulting in a lack of economic viability for recovery. Furthermore, the iron yield decreased significantly to only 78.9%. In this embodiment of the invention, while ensuring an iron yield >90%, the concentration of sulfuric acid generated is significantly increased. The mass concentration of H2SO4 in the first filtrate can reach 33%-37%, a significant improvement compared to the comparative example. The resulting sulfuric acid can be directly used to react with reduced ilmenite to prepare synthetic rutile without concentration, achieving sulfuric acid recovery. Alternatively, it can be concentrated and reused in the sulfuric acid process for the acid hydrolysis of titanium dioxide. Compared to the comparative example, the significantly increased concentration of sulfuric acid significantly reduces concentration costs and improves economic viability for recovery.

[0045] Although the present invention has been illustrated and described with specific embodiments, it should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; those skilled in the art should understand that modifications can be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein, without departing from the spirit and scope of the present invention; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention; therefore, this means that all such substitutions and modifications that fall within the scope of the present invention are included in the appended claims.

Claims

1. A method for co-producing ferrous oxalate dihydrate and sulfuric acid, characterized in that, Includes the following steps: S1. Mix solid FeSO4·7H2O and solid H2C2O4·2H2O, and stir the mixture at 70~80℃ using the crystal water released in situ from the raw materials as the reaction medium for 2~4 hours to obtain a slurry; S2. While the slurry is still hot, a first solid-liquid separation is performed to obtain a first filtrate and a ferrous oxalate dihydrate filter cake; S3. The ferrous oxalate dihydrate filter cake is washed and dried with hot water to obtain ferrous oxalate dihydrate; After cooling and allowing the first filtrate to stand, a second solid-liquid separation is performed to obtain sulfuric acid and unreacted solid raw materials.

2. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1, characterized in that, In step S3, after the second solid-liquid separation, the following steps are also included: multi-effect evaporation concentration and third solid-liquid separation are performed on the sulfuric acid to obtain concentrated sulfuric acid.

3. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1 or 2, characterized in that, In step S1, FeSO4·7H2O is a byproduct generated during the sulfuric acid process for titanium dioxide production.

4. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1 or 2, characterized in that, In step S1, the molar ratio of FeSO4·7H2O to H2C2O4·2H2O is 1:(0.95~1.05); and / or, the stirring speed is 200~800 rpm.

5. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1 or 2, characterized in that, In step S2, the temperature of the first solid-liquid separation is ≥60℃.

6. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1 or 2, characterized in that, In step S2, the mass concentration of H2SO4 in the first filtrate is 33%~37%.

7. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1 or 2, characterized in that, In step S3, the temperature of the hot water is 50~60℃, and the filter cake is washed until the conductivity of the washing liquid is ≤1000µS / cm.

8. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1 or 2, characterized in that, In step S3, the first filtrate is cooled to 0~10℃ and then allowed to stand for 2~6 hours; and / or, the drying temperature is 60~80℃ and the drying time is 2~8 hours.

9. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 1 or 2, characterized in that, In step S3, the unreacted solid raw material obtained from the second solid-liquid separation is returned to step S1 for recycling.

10. The method for co-producing ferrous oxalate dihydrate and sulfuric acid according to claim 2, characterized in that, The concentrated sulfuric acid contains H2SO4 at a mass concentration of ≥55%; and / or the concentrated sulfuric acid contains iron at a content of ≤3000ppm.