A method for purifying and refining crystallization of titanium dioxide by-product ferrous sulfate

Abstract

The application discloses a purification and refining crystallization method of titanium white by-product ferrous sulfate, which comprises the following steps: (1) under the stirring condition, dissolving the titanium white raw material containing the by-product ferrous sulfate in a mixed solution of ferrous sulfate heptahydrate and water at 60-80 DEG C, then adding iron powder for reduction, adding acid to adjust the pH to 1-3, adding a flocculating agent for flocculation, and obtaining a clear liquid; (2) under the stirring condition, cooling and crystallizing the clear liquid obtained in the step (1) in three stages, and carrying out solid-liquid separation to obtain a primary crystallization crude product; and (3) under the stirring condition, dissolving the primary crystallization crude product obtained in the step (2) in water at 60-80 DEG C, cooling and crystallizing in three stages, and obtaining the titanium white by-product ferrous sulfate. The preparation process can continuously, stably, efficiently and in large batches prepare the ferrous sulfate heptahydrate product, so as to be used for preparing a battery-grade ferrous oxalate.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of crystallization technology in chemical engineering industry, and particularly relates to a purification and crystallization method for titanium dioxide by-product ferrous sulfate. BACKGROUND

[0002] Ferrous sulfate heptahydrate (CAS: 7782-63-0), ferrous sulfate, also known as green vitriol, is an important by-product in the production of titanium dioxide, and is widely used in the fields of chemical industry and metallurgy. According to the calculation that about 3 tons of ferrous sulfate are produced per ton of titanium dioxide, nearly 2.5 million tons of ferrous sulfate heptahydrate will be produced annually. In order to improve resource utilization, ferrous sulfate can also be used as an iron source material for preparing lithium ion battery cathode material lithium iron phosphate by a liquid phase method. However, the by-product ferrous sulfate from titanium dioxide has high impurity content and low purity, and direct use in the preparation of battery-grade ferrous oxalate will have a great impact on the performance of the subsequent lithium iron phosphate cathode material. Therefore, before comprehensive utilization, the by-product ferrous sulfate needs to be purified.

[0003] At present, as an important separation and purification method, crystallization is widely used in the fields of food, medicine, fine chemical industry, etc., but there are still few systematic studies on the purification of by-product ferrous sulfate by crystallization. Crystallization has the advantages of low cost, clean production and high separation efficiency, and is particularly suitable for the purification of ferrous sulfate. Therefore, by processing the by-product ferrous sulfate from titanium dioxide into battery-grade ferrous oxalate, not only the serious stacking problem of by-product ferrous sulfate can be solved, but also the development needs of the battery industry can be met.

[0004] At present, there are few studies on the continuous production of ferrous sulfate heptahydrate. Most of the research focuses on the removal rules and effects of impurities (such as Ti, Mg and Mn), but for industrialization and large-scale production, only continuous process can meet the project goals. This study focuses on the key parameters and process optimization of the continuous production of ferrous sulfate heptahydrate. SUMMARY

[0005] In order to overcome the problem that the by-product ferrous sulfate heptahydrate from titanium dioxide has too high Mg content and cannot be used to prepare battery-grade ferrous oxalate, and to solve the lack of continuous production theory in the production process, the application provides a purification and crystallization method for titanium dioxide by-product ferrous sulfate. The yield of ferrous sulfate heptahydrate crystals prepared by the continuous crystallization method can reach more than 80%, the particle size is 150-250 microns, it is not easy to coalesce, and the content of impurity Mg is less than 500 ppm, which meets the application in the preparation of battery-grade ferrous oxalate.

[0006] The technical scheme of the application is as follows:

[0007] One of the purposes of the present application is to provide a purification and refining crystallization method of ferrous sulfate by-product of titanium dioxide, the flow of the purification and refining crystallization is shown in the attached Figure 1 The method comprises the following steps:

[0008] (1) Under stirring, dissolve the titanium dioxide raw material containing by-product ferrous sulfate in water at 60-80℃ (for example, 60℃, 65℃, 70℃, 75℃, 80℃, etc.) to prepare a ferrous sulfate heptahydrate-water mixed solution, then add iron powder for reduction, adjust the pH to 1-3 by adding acid, add a flocculating agent for flocculation, and obtain a clear liquid;

[0009] (2) Under stirring, once reduce the clear liquid obtained in step (1) to 50-60℃ (for example, 50℃, 55℃, 60℃, etc.), continuously add crystal seeds, and perform secondary reduction to 30-35℃ (for example, 30℃, 32℃, 35℃, etc.) for 50-70 min, and then perform three times of reduction to 20-25℃ (for example, 20℃, 22℃, 25℃, etc.) for 50-70 min, solid-liquid separation, and obtain a primary crystallization crude product;

[0010] (3) Under stirring, dissolve the primary crystallization crude product obtained in step (2) in water at 60-80℃ (for example, 60℃, 65℃, 70℃, 75℃, 80℃, etc.), once reduce to 50-60℃ (for example, 50℃, 55℃, 60℃, etc.), continuously add crystal seeds, and perform secondary reduction to 30-35℃ (for example, 30℃, 32℃, 35℃, etc.) for 50-70 min, and then perform three times of reduction to 20-25℃ (for example, 20℃, 22℃, 25℃, etc.) for 50-70 min, and obtain the ferrous sulfate by-product of titanium dioxide.

[0011] In the present application, since ferrous sulfate heptahydrate has the characteristic that the solubility changes greatly with the increase of temperature, the purification can be carried out by cooling crystallization. However, the production of industrial by-product ferrous sulfate is huge in China, and the conventional batch crystallization method cannot meet the needs of enterprises, so the continuous crystallization method is considered to efficiently and stably purify the titanium dioxide by-product on a large scale. Therefore, the entire crystallization process, including the temperature, residence time, feed rate, and waste liquid treatment of each stage, needs to be strictly controlled to maintain the stability of the entire process. Therefore, the ferrous sulfate heptahydrate-water solution with a temperature of 60-80℃ and a ferrous sulfate heptahydrate concentration of 1.3-2.0 g / mL is continuously cooled and crystallized through three-stage crystallizers (the temperature of each stage is 50-60℃, 30-35℃, and 20-25℃, respectively), the content of impurity Mg is successfully reduced to below 500 ppm through the two-stage crystallization process, and the recovery rate of the entire process is above 80% by recycling 50-70% of the mother liquor from the first-stage crystallization and using the mother liquor from the second-stage crystallization to dissolve the titanium dioxide by-product raw material. This study determines the optimal process by changing the temperature and residence time of each stage, and significantly improves the yield by excellent utilization of the mother liquor, which has good guiding significance for industrial production.

[0012] Preferably, in steps (1), (2) and (3), the stirring rate is 200-400 rpm, such as 200 rpm, 300 rpm, 400 rpm, etc.; in the present application, the particle size of ferrous sulfate heptahydrate can be adjusted by changing the stirring rate in combination with other process conditions, so that the average particle size of the product is controlled within 150-250 μm; if the stirring rate is not within the range defined in the present application, it may cause uneven crystal size distribution or the problem of explosive nucleation, resulting in a significant increase in the content of impurity magnesium in the product.

[0013] Preferably, in step (1), the concentration of ferrous sulfate heptahydrate in the mixed solution is 1.30-2.00 g / mL, such as 1.3 g / mL, 1.4 g / mL, 1.5 g / mL, 1.6 g / mL, 1.7 g / mL, 1.8 g / mL, 1.9 g / mL, 2.0 g / mL, etc.; in the present application, if the concentration of the ferrous sulfate heptahydrate-water solution is too high, the content of Mg in the final product will be too high; if the concentration is too low, the final yield will be too low, causing economic problems.

[0014] Preferably, the acid in step (1) is dilute sulfuric acid, which is obtained by mixing 98% concentrated sulfuric acid and water in a volume ratio of (1-4):(9-6).

[0015] Preferably, the amount of iron added in step (1) is 1-3 g based on 100 g of titanium dioxide raw material containing by-product ferrous sulfate.

[0016] Preferably, the flocculating agent in step (1) is polyacrylamide.

[0017] Preferably, the amount of the flocculating agent in step (1) is 0.1-0.3g, based on 100g of the titanium dioxide raw material containing by-product ferrous sulfate.

[0018] Preferably, the particle size of the crystal seeds in steps (2) and (3) is 100-200μm.

[0019] Preferably, the amount of the crystal seeds in step (2) is 1 / 4-1 / 3 of the mass of the final ferrous sulfate heptahydrate.

[0020] Preferably, the liquid obtained by solid-liquid separation in step (2) is recycled, which includes rotary evaporation of the liquid, and rotary evaporation of 50-70% of the mother liquor for the dissolution of the titanium dioxide raw material containing by-product ferrous sulfate in step (1).

[0021] Preferably, the temperature of the rotary evaporation is 50-70℃, and the rotation speed of the rotary evaporation is 60-100rpm.

[0022] Preferably, step (3) further comprises solid-liquid separation, washing and drying of the mixture obtained after the third cooling.

[0023] Preferably, the solid-liquid separation is filtration.

[0024] Preferably, the washing is washing with saturated ferrous sulfate heptahydrate solution for 2-4 times.

[0025] Preferably, the drying is vacuum drying, wherein the vacuum degree is 0.95bar, the drying temperature is 40-50℃, and the drying time is 12-16h.

[0026] Preferably, the preparation method further comprises rotary evaporation of the liquid obtained by solid-liquid separation, and rotary evaporation of 50-70% of the mother liquor for the dissolution of the titanium dioxide raw material containing by-product ferrous sulfate in step (1).

[0027] Preferably, the temperature of the rotary evaporation is 50-70℃, and the rotation speed of the rotary evaporation is 60-100rpm.

[0028] In step (4), the solution after rotary evaporation is cooled to 20-25℃ and filtered to obtain the rotary evaporation product. Then, the product is mixed with a new batch of raw material to repeat the crystallization process twice.

[0029] The second object of the present application is to prepare ferrous sulfate heptahydrate crystals by the purification and crystallization process described in the first object; preferably, the content of magnesium in the ferrous sulfate heptahydrate crystals is less than 500ppm.

[0030] Preferably, the particle size of the ferrous sulfate heptahydrate crystals is 150-250 μm.

[0031] The third object of the present application is the use of the ferrous sulfate heptahydrate crystals obtained in the second object in the preparation of battery-grade ferrous oxalate.

[0032] The above method has the following beneficial effects:

[0033] a) The particle size of the ferrous sulfate heptahydrate product obtained by the method is uniform, the coalescence phenomenon is well inhibited, the explosive nucleation is successfully inhibited, and the purity of the product is greatly improved.

[0034] b) The method efficiently and stably ensures the continuity of the process, can be used to guide industrial scale-up production, and can be used to produce large-scale ferrous sulfate heptahydrate products with standard purity.

[0035] c) The method effectively utilizes the mother liquor of the primary and secondary crystallization processes, which can increase the yield of the process and reduce the discharge of waste liquid and the economic problems of waste liquid treatment. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 Flow chart of primary crystallization of titanium dioxide by-product ferrous sulfate;

[0037] Figure 2 The impurity removal limit (Mg content before washing) of the primary and secondary crystallization under different feed concentrations;

[0038] Figure 3 Scanning electron microscope photograph of the ferrous sulfate heptahydrate crystals prepared in Example 1 (scale: 500 μm). DETAILED DESCRIPTION

[0039] Example 1:

[0040] As shown in Figure 1 :

[0041] (1) At 70°C, prepare a mixed solution of ferrous sulfate heptahydrate and water with a concentration of 1.55 g / ml, stir until the titanium dioxide by-product is completely dissolved in water, then add Fe powder for reduction, add dilute sulfuric acid to adjust the pH to 1-3, and add polyacrylamide flocculant for flocculation.

[0042] (2) The solution is cooled to 50°C, and a large amount of seed crystals (1 / 3 of the product) is continuously stirred; then it is instantaneously cooled to 32°C and continuously stirred for a period of time, and then it is instantaneously cooled to 20°C and continuously stirred for a period of time, and the product is obtained by filtration, and the primary crystallization crude product is obtained by washing with a saturated solution of ferrous sulfate heptahydrate.

[0043] (3) The product obtained in step (2) is dissolved at 70°C to prepare a ferrous sulfate heptahydrate-water mixture solution with a ferrous sulfate heptahydrate concentration of 1.55 g / mL, cooled to 50°C, and a large amount of crystal seeds (1 / 3 product) is added while continuously stirring; then instantaneously cooled to 32°C and continuously stirred for a period of time, and then instantaneously cooled to 20°C and continuously stirred for a period of time, and the product is filtered, and washing with a ferrous sulfate heptahydrate saturated solution can obtain a secondary crystallization product;

[0044] (4) The mother liquor obtained in step (2) is recovered by rotary evaporation, and the rotary evaporation mother liquor amount is 65%, and the mother liquor obtained in step (3) is used for the dissolution of the titanium dioxide byproduct raw material in step (1), and the ferrous sulfate heptahydrate concentration is kept unchanged; the impurity Mg content of the product after two times of crystallization is as shown in Table 1 (corresponding to a 55°C saturated solution), and it can be seen that the required standard of Mg content of 412 ppm (after washing) can be reached; as shown in Table 2, the prepared ferrous sulfate heptahydrate product has uniform particle size, and the coalescence phenomenon is well inhibited, and the average particle size of the product is about 200 microns. Figure 2 Figure 3

[0045] Example 2:

[0046] (1) At 70°C, a ferrous sulfate heptahydrate-water mixture solution with a concentration of 1.75 g / ml is configured, and stirring is performed until the titanium dioxide byproduct is completely dissolved in water, and then Fe powder is added for reduction, and dilute sulfuric acid is added to adjust the PH to 1-3, and a polyacrylamide flocculant is flocculated;

[0047] (2) The solution is cooled to 50°C, and a large amount of crystal seeds (1 / 3 product) is added while continuously stirring; then instantaneously cooled to 30°C and continuously stirred for a period of time, and then instantaneously cooled to 22°C and continuously stirred for a period of time, and the product is filtered, and washing with a ferrous sulfate heptahydrate saturated solution can obtain a primary crystallization crude product.

[0048] (3) The product obtained in step (2) is dissolved at 70°C to prepare a ferrous sulfate heptahydrate-water mixture solution with a ferrous sulfate heptahydrate concentration of 1.55 g / mL, cooled to 50°C, and a large amount of crystal seeds (1 / 3 product) is added while continuously stirring; then instantaneously cooled to 30°C and continuously stirred for a period of time, and then instantaneously cooled to 22°C and continuously stirred for a period of time, and the product is filtered, and washing with a ferrous sulfate heptahydrate saturated solution can obtain a secondary crystallization product;

[0049] (4) The mother liquor obtained in step (2) is recovered by rotary evaporation, and the rotary evaporation mother liquor amount is 50%, and the mother liquor obtained in step (3) is used for the dissolution of the titanium dioxide byproduct raw material in step (1), and the ferrous sulfate heptahydrate concentration is kept unchanged; the impurity Mg content of the product after two times of crystallization is as shown in Table 1 (corresponding to a 55°C saturated solution), and it can be seen that the required standard of Mg content of 412 ppm (after washing) can be reached; as shown in Table 2, the prepared ferrous sulfate heptahydrate product has uniform particle size, and the coalescence phenomenon is well inhibited, and the average particle size of the product is about 200 microns. Figure 2 ​​As shown (corresponding to 60°C saturated solution), it can be seen that the required standard can be reached, i.e. the Mg content is 488 ppm (after washing).

[0050] The prepared ferrous sulfate heptahydrate product has uniform particle size and good inhibition of coalescence, and the average particle size of the product is about 220 microns.

[0051] Example 3:

[0052] (1) At 70°C, a ferrous sulfate heptahydrate-water mixed solution with a concentration of 1.4 g / ml is prepared, stirring until the titanium dioxide by-product is completely dissolved in water, then adding Fe powder for reduction, adding dilute sulfuric acid to adjust the pH to 1-3, and adding polyacrylamide flocculant for flocculation;

[0053] (2) The solution is cooled to 55°C, and a large amount of crystal seeds (1 / 4 product) is continuously stirred; then it is instantaneously cooled to 35°C and continuously stirred for a period of time, then it is instantaneously cooled to 25°C and continuously stirred for a period of time, and the product is obtained by filtration, and the primary crystallization crude product can be obtained by washing with a saturated ferrous sulfate heptahydrate solution.

[0054] (3) The product obtained in step (2) is dissolved at 70°C to prepare a ferrous sulfate heptahydrate-water mixed solution with a concentration of 1.4 g / ml, cooled to 55°C, and a large amount of crystal seeds (1 / 4 product) is continuously stirred; then it is instantaneously cooled to 35°C and continuously stirred for a period of time, then it is instantaneously cooled to 25°C and continuously stirred for a period of time, and the product is obtained by filtration, and the secondary crystallization product can be obtained by washing with a saturated ferrous sulfate heptahydrate solution.

[0055] (4) The mother liquor obtained in step (2) is recovered by rotary evaporation, and the amount of the rotary evaporation mother liquor is 55%, and the mother liquor obtained in step (3) is used for the dissolution of the titanium dioxide by-product raw material in step (1), and the concentration of ferrous sulfate heptahydrate is kept unchanged; the impurity Mg content of the product after two times of crystallization is as shown in Table 2 (corresponding to a 52°C saturated solution), it can be seen that the required standard can be reached, i.e. the Mg content is 400 ppm (after washing). Figure 2

[0056] The prepared ferrous sulfate heptahydrate product has uniform particle size and good inhibition of coalescence, and the average particle size of the product is about 190 microns.

[0057] Example 4:

[0058] (1) At 70°C, a ferrous sulfate heptahydrate-water mixed solution with a concentration of 1.55 g / ml is prepared, stirring until the titanium dioxide by-product is completely dissolved in water, then adding Fe powder for reduction, adding dilute sulfuric acid to adjust the pH to 1-3, and adding polyacrylamide flocculant for flocculation;

[0059] ​(2) The solution is cooled to 60°C, and a large amount of seed crystal (1 / 4 product) is added while stirring; then it is instantaneously cooled to 30°C and stirred for a period of time, and then it is instantaneously cooled to 22°C and stirred for a period of time, and the product is filtered, and the primary crystallization crude product is obtained by washing with a saturated ferrous sulfate heptahydrate solution.

[0060] (3) The product obtained in step (2) is dissolved at 70°C to obtain a ferrous sulfate heptahydrate-water mixed solution with a ferrous sulfate heptahydrate concentration of 1.55 g / mL, and it is cooled to 60°C, and a large amount of seed crystal (1 / 4 product) is added while stirring; then it is instantaneously cooled to 30°C and stirred for a period of time, and then it is instantaneously cooled to 22°C and stirred for a period of time, and the product is filtered, and the secondary crystallization product is obtained by washing with a saturated ferrous sulfate heptahydrate solution;

[0061] (4) The mother liquor obtained in step (2) is recovered by rotary evaporation, and the rotary evaporation mother liquor amount is 50%, and the mother liquor obtained in step (3) is used for the dissolution of the titanium dioxide byproduct raw material in step (1), and the ferrous sulfate heptahydrate concentration is kept unchanged; the impurity Mg content of the product after two times of crystallization is shown in Table 1 (corresponding to a 55°C saturated solution), and it can be seen that the required standard, i.e., the Mg content of 450 ppm (after washing), can be reached. Figure 2

[0062] The prepared ferrous sulfate heptahydrate product has uniform particle size, and the coalescence phenomenon is well inhibited, and the average particle size of the product is about 240 microns.

[0063] Comparative Example 1:

[0064] The difference from Example 1 is only that the initial concentration of the solution is 2.5 mg / ml.

[0065] The product obtained has a serious coalescence phenomenon, the particle size is not uniform, the CSD distribution is wide, and the Mg content after secondary crystallization is 1500 ppm.

[0066] Comparative Example 2:

[0067] The difference from Example 1 is only that the residence time of each stage is 30 min.

[0068] The residence time is too short, resulting in insufficient time for the growth of the crystal, and the average particle size of the product is small, i.e., 150 microns, a large amount of nucleation phenomenon occurs due to excessive supersaturation, and the coalescence phenomenon is obvious, and the Mg content after secondary crystallization is 1200 ppm.

[0069] Comparative Example 3:

[0070] The difference from Example 1 is only that the temperature of the middle crystallizer in the three-stage crystallizer is 28°C.

[0071] ​The product has serious agglomeration, uneven particle size, wide CSD distribution, and the Mg content of the secondary crystallization product is 850 ppm.

[0072] Comparative Example 4:

[0073] The difference from Example 1 is only that the amount of seed crystals added is 1 / 5 of the product.

[0074] The remaining supersaturation after the consumption of the crystal growth causes the secondary nucleation of the solution, resulting in agglomeration, and the Mg content of the secondary crystallization product is 900 ppm.

[0075] Comparative Example 5:

[0076] The difference from Example 1 is only that the particle size of the seed crystals added is 100 microns.

[0077] The particle size that is too small affects the solubility of the crystal, deviates from the initial measured thermodynamics, and after the addition of seed crystals, part of the solubility is dissolved, so that the remaining supersaturation after the consumption of the crystal growth causes the secondary nucleation of the solution, resulting in agglomeration, and the Mg content of the secondary crystallization product is 760 ppm.

[0078] Comparative Example 6:

[0079] The difference from Example 1 is only that the temperature at the end of the third crystallizer is 10°C.

[0080] The product purity can meet the requirements, but it is too low for industrial production. The temperature of 10°C is relatively low, and a large amount of steam is consumed to reduce the temperature to such a low temperature during the scale-up production, and scaling problems are prone to occur. Therefore, the temperature of the last set of the third crystallizer is set to more than 20°C for consideration of heat exchange and economy.

[0081] From the comparison of Example 1 and Comparative Example 1, when the initial concentration of the solution is lower than the limited range of the application, because the distribution coefficient of the solvent is certain, a certain amount of solvent cannot handle more amount of Mg impurities, so more Mg is present in the product. Therefore, the feed amount cannot be increased for the pursuit of too high yield.

[0082] From the comparison of Example 1 and Comparative Example 2, when the residence time of the solution in each crystallizer is not within the limited range of the application, it will result in smaller crystal particle size, so that the supersaturation is used for nucleation rather than growth or unnecessary residence time increases the process cost.

[0083] From Example 1 and Comparative Example 3, when the temperature of each crystallizer is not within the limited range of the application, it will result in serious agglomeration of the product, uneven particle size, wide CSD distribution, and high impurity content of the product.

[0084] From the comparison of Example 1 and Comparative Examples 4 and 5, it can be seen that when the amount of seed crystals or the particle size of the seed crystals is not within the defined range of the present application, the supersaturation generated by the solution will be consumed in the growth of the crystals and still have a remainder, resulting in secondary nucleation of the solution, thereby causing coalescence, and thus high impurity content of the product.

[0085] From the comparison of Example 1 and Comparative Example 6, it can be seen that a temperature of 10℃ is relatively low for industrial production, and in the process of scaling up production, a large amount of steam will be consumed to reduce the temperature to such a low temperature, and at the same time, fouling problems are likely to occur, so the temperature of the last set of the three-stage crystallizer is set to be higher than 20℃ for consideration of heat exchange and economy.

[0086] The present application discloses and proposes a new crystallization technology for purifying and refining ferrous sulfate by-product of titanium dioxide, and those skilled in the art can realize the present application by referring to the content of the present application, appropriately changing the initial concentration, residence time of each stage of the crystallizer, temperature of each set of the crystallizer, stirring rate, rotary evaporation percentage, seed crystal addition amount and particle size, etc. The method of the present application has been described through preferred examples, and relevant skilled persons can obviously modify or appropriately change and combine the method and product described in the present application without departing from the content, spirit and scope of the present application, to realize the present application technology. It is particularly important to point out that all similar replacements and modifications are obvious to those skilled in the art, and they are all considered to be included in the spirit, scope and content of the present application.

Claims

1. A purification and refining crystallization method of titanium white by-product ferrous sulfate, characterized by, The purification and refining crystallization method comprises the following steps: (1) Dissolve the titanium dioxide raw material containing by-product ferrous sulfate in water at 60-80℃ under stirring to prepare a ferrous sulfate heptahydrate-water mixed solution, then add iron powder for reduction, add acid to adjust pH to 1-3, add flocculant for flocculation to obtain a clear solution; (2) Under stirring, the clear solution obtained in step (1) is once cooled to 50-60℃, crystal seeds are continuously added, and then twice cooled to 30-35℃ for 50-70min, and then thrice cooled to 20-25℃ for 50-70min, and then solid-liquid separation is performed to obtain a primary crystallization crude product; (3) Under stirring, the primary crystallization crude product obtained in step (2) is dissolved in water at 60-80℃, once cooled to 50-60℃, and crystal seeds are continuously added, and then twice cooled to 30-35℃ for 50-70min, and then thrice cooled to 20-25℃ for 50-70min to obtain the titanium dioxide by-product ferrous sulfate; The particle size of the crystal seeds in steps (2) and (3) is 100-200μm; The addition amount of the crystal seeds in steps (2) and (3) is 1 / 4-1 / 3 of the mass of the obtained ferrous sulfate heptahydrate.

2. The purification and refining crystallization process of claim 1, wherein, In steps (1), (2) and (3), the stirring rate is 200-400rpm.

3. The purification and refining crystallization process of claim 1, wherein, The concentration of ferrous sulfate heptahydrate in the mixed solution in step (1) is 1.30-2.00g / mL.

4. The purification and refining crystallization process of claim 1, wherein, The acid in step (1) is dilute sulfuric acid, wherein the dilute sulfuric acid is obtained by mixing 98% concentrated sulfuric acid and water according to a volume ratio of (1-4):(9-6).

5. The purification and refining crystallization process of claim 1, wherein, The addition amount of iron in step (1) is 1-3g, based on 100g of the titanium dioxide raw material containing by-product ferrous sulfate.

6. The purification and refining crystallization process of claim 1, wherein, The flocculant in step (1) is polyacrylamide.

7. The purification and refining crystallization process of claim 1, wherein, The addition amount of the flocculant in step (1) is 0.1-0.3g, based on 100g of the titanium dioxide raw material containing by-product ferrous sulfate.

8. The purification and refining crystallization process of claim 1, wherein, The liquid obtained by solid-liquid separation in step (2) is recycled, and the recycling comprises rotary evaporation of the liquid, and rotary evaporation of 50-70% of the mother liquor is used for the dissolution of the titanium dioxide raw material containing by-product ferrous sulfate in step (1).

9. The purification and refining crystallization process of claim 8, wherein, The temperature of the rotary evaporation is 50-70℃, and the rotation speed of the rotary evaporation is 60-100rpm.

10. The purification and refining crystallization process of claim 1, wherein, Step (3) further comprises solid-liquid separation, washing and drying of the mixture obtained after thrice cooling.

11. The purification and refining crystallization process of claim 10, wherein, The solid-liquid separation is filtration.

12. The purification and refining crystallization process of claim 10, wherein, The washing is washing with saturated ferrous sulfate heptahydrate saturated solution for 2-4 times.

13. The purification and refining crystallization process of claim 10, wherein, The drying is vacuum drying, wherein the vacuum degree is 0.95bar, the drying temperature is 40-50℃, and the drying time is 12-16h.

14. The purification and refining crystallization process of claim 10, wherein, The liquid obtained by solid-liquid separation in step (3) is recycled for the dissolution of the titanium dioxide by-product raw material in step (1).

15. The purification and refining crystallization process of claim 14, wherein, The concentration of ferrous sulfate heptahydrate in the solution after the dissolution of the titanium dioxide by-product raw material is consistent with that before the dissolution.

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