Countercurrent crystallization production process for reducing impurity content in ferrous sulfate heptahydrate

Abstract

The invention discloses a countercurrent crystallization production process for reducing the content of impurities such as magnesium (Mg), manganese (Mn) and titanium (Ti) in ferrous sulfate heptahydrate, and belongs to the technical field of purification of inorganic compounds. According to the process, through two-stage design of'system initialization and first-batch material treatment 'and'continuous countercurrent crystallization production', by utilizing cooperative countercurrent cooperation of a dissolving kettle, a crystallization kettle, a centrifugal machine and a multi-stage mother liquor tank, cascade recycling of mother liquor and repeated recrystallization and purification of the materials are realized. According to the process, thousands of ppm of Mg, fifty hundred ppm of Mn and forty ppm of Ti in the ferrous sulfate heptahydrate crude product can be reduced to be smaller than or equal to 180 ppm, smaller than or equal to 40 ppm and smaller than or equal to 25 ppm respectively, meanwhile, the solvent water consumption is reduced by 40% or above, mother liquor emission is reduced, and the process has the advantages of being high in industrial continuity and automation degree and suitable for large-scale production of the low-impurity ferrous sulfate heptahydrate product.

Description

Technical Field

[0001] This invention relates to the field of compound crystallization and purification technology, specifically to a countercurrent crystallization process for ferrous sulfate heptahydrate, which is particularly suitable for efficiently removing Mg, Mn, and Ti impurities from ferrous sulfate heptahydrate. Background Technology

[0002] Ferrous sulfate heptahydrate is an important inorganic compound used in agricultural fertilizers, chemical catalysts, and other fields, and is also widely used in the pharmaceutical and food industries. Its impurity content (especially Mg, Mn, and Ti) directly affects product performance and application effectiveness. Traditional multiple recrystallization processes suffer from problems such as high solvent (water) consumption, low recrystallization yield, cumbersome operation, and limited removal of Mg, Mn, and Ti impurities. Conventional non-countercurrent crystallization cannot efficiently utilize the mother liquor solute, resulting in low yield, resource waste, and difficulty in meeting the industrial requirements of the food and pharmaceutical industries for low-impurity products (Mg ≤ 200 ppm, Mn ≤ 50 ppm, Ti ≤ 30 ppm). Therefore, there is an urgent need for a crystallization process with high solvent utilization, high product yield, significant removal of Mg, Mn, and Ti impurities, and low mother liquor discharge. Summary of the Invention

[0003] To overcome the shortcomings of existing technologies, such as high solvent consumption, insufficient impurity removal, low product yield, and large amounts of mother liquor discharge, which make large-scale industrial production difficult, this invention provides a countercurrent crystallization process for reducing the content of Mg, Mn, and Ti impurities in ferrous sulfate heptahydrate. Through a two-stage countercurrent design and cascaded reuse of mother liquor, the dual goals of efficient impurity removal and resource conservation are achieved.

[0004] The process includes system initialization and initial material handling, as well as continuous countercurrent crystallization production. The specific steps are as follows: (I) System initialization and initial material processing stage This stage is used to establish a multi-stage mother liquor storage system to obtain low-impurity starting wet solids that meet the requirements of continuous production, laying the foundation for the impurity gradient in subsequent continuous countercurrent production. Specifically, this includes: 1. Initial crystallization: The crude feed containing ferrous sulfate heptahydrate (Mg content > 2000 ppm, Mn content > 800 ppm, Ti content > 100 ppm) is fed into a stainless steel jacketed dissolving reactor. Furthermore, the dissolving vessel is equipped with an anchor-type mechanical stirring device, with the stirring speed set to 30~500rpm to ensure uniform mixing of materials; saturated steam at 0.3~0.6MPa is introduced through the steam inlet to raise the temperature to 60~100℃, and condensate is discharged through the condensate outlet. Stirring is started to completely dissolve the materials.

[0005] Furthermore, the temperature is maintained for 1~6 hours. After dissolution, the material is pumped from the outlet of the dissolution vessel into the stainless steel jacketed crystallizer through the first transfer pump. Furthermore, during the transfer process, it is ensured that the liquid is transported smoothly and without the generation of bubbles. Chilled water at -10~10℃ is introduced through the chilled water inlet to cool the temperature to 5~55℃ for crystallization for 1~6 hours. The chilled water is discharged from the chilled water outlet.

[0006] Furthermore, the cooling rate is controlled at 5~10℃ / h, and the stirring speed of the crystallizer is 30~500rpm to promote crystal growth; after solid-liquid separation by a plate centrifuge, the liquid flows by gravity from the centrifuge mother liquor outlet to the first mother liquor tank.

[0007] Furthermore, the separation time is controlled at 5~60min to ensure that the moisture content of the wet solids is ≤20%; the wet solids are transported to the dissolving tank by conveyor belt through the solid outlet of the centrifuge, and the mother liquor is pumped into the first mother liquor tank for storage by the second transfer pump.

[0008] Furthermore, the first mother liquor tank is equipped with a low-temperature insulation layer to maintain the temperature of the mother liquor at 5~15℃ to reduce solute precipitation.

[0009] 2. Second to fifth crystallization: During the nth crystallization (n=2,3,4,5), add fresh deionized water or distilled water to the dissolving vessel.

[0010] Furthermore, the fresh water needs to undergo ion exchange treatment to avoid introducing new impurities; the mass-to-volume ratio of the crude product dry basis to the solvent water is controlled at 1:1.5~1:3 (g:mL).

[0011] Furthermore, the dissolving vessel is equipped with an anchor-type mechanical stirring device, with the stirring speed set to 30~500 rpm to ensure uniform mixing of materials; 0.3~0.6MPa saturated steam is introduced to raise the temperature to 60~100℃, and stirring is started to completely dissolve the materials.

[0012] Furthermore, maintain the temperature for 1-6 hours, and after dissolution, pump the solution into the jacketed crystallizer through the first transfer pump; then introduce -10-10℃ chilled water or plant circulating water to cool the temperature to 5-55℃ for 1-6 hours for crystallization.

[0013] Furthermore, the cooling rate is controlled at 1~15℃ / h, and the stirring speed of the crystallizer is 30~500rpm to promote crystal growth; solid-liquid separation is performed by centrifugation.

[0014] Furthermore, the separation time is controlled at 5~60 min to ensure that the moisture content of the wet solids is ≤20%; Furthermore, the liquid-solid ratio is dynamically adjusted based on the moisture content of the wet solid from the previous crystallization. For every 5% increase in moisture content, the amount of solvent water increases by approximately 10%. The "crystallization-separation" operation is repeated after dissolving the returned wet solid.

[0015] Furthermore, the mother liquor is pumped into the nth mother liquor tank by the (n+1)th transfer pump.

[0016] Furthermore, the mother liquor tank is equipped with a low-temperature insulation layer to maintain the temperature of the mother liquor at 5~15℃ to reduce solute precipitation.

[0017] Furthermore, after each return, the wet solid needs to be sampled and tested to record the changing trends of Mg, Mn, and Ti impurity content. After five crystallizations, the initial wet solid with Mg≤180ppm, Mn≤40ppm, and Ti≤25ppm is obtained, completing the system initialization. Furthermore, the criterion for completing the initialization is that the fluctuation of the wet solid impurity content is ≤5ppm after two consecutive crystallizations to ensure system stability.

[0018] (II) Continuous Countercurrent Crystallization Production Stage This stage is a continuous industrial production process, in which the targeted enrichment and removal of impurities are achieved through countercurrent reuse of the mother liquor, forming a stable impurity gradient cycle. Specifically, it includes: 1. First-stage crystallization: The new crude product is added to the dissolving tank.

[0019] Further, the new crude product is added to solvent water at a mass-volume ratio of 1:1.5 to 1:3 (g:mL) to dissolve the wet solid, followed by crystallization and separation.

[0020] Furthermore, the temperature of the dissolving vessel is controlled at 60~100℃ and maintained for 1~6 hours. After dissolution, the solution is transferred to the crystallization vessel.

[0021] Furthermore, the crystallization temperature is controlled at 5~55℃ and maintained for 1~6h. This temperature range allows for effective separation of the target crystal from impurities. The wet solid is then separated by centrifugation and returned to the dissolving vessel.

[0022] Furthermore, a closed conveyor belt is used during the wet solids transportation process to prevent oxidation from contact with air; the mother liquor is pumped into the first mother liquor tank by the second transfer pump and can be discharged through the seventh transfer pump to balance impurities.

[0023] 2. Second-stage crystallization: The mother liquor in the third mother liquor tank is pumped into the dissolving vessel by the ninth transfer pump.

[0024] Furthermore, the temperature of the dissolving vessel is controlled at 60~100℃ and maintained for 1~6 hours. After dissolution, the solution is transferred to the crystallization vessel. Further, the crystallization temperature is controlled at 5~55℃ and maintained for 1~6 hours. This temperature range allows for effective separation of the target crystals from impurities. The solids are then separated by centrifugation and returned to the dissolving vessel. Furthermore, during the dissolution process, the stirring speed is increased to 30-500 rpm to promote full contact between the mother liquor and the solid.

[0025] Furthermore, the mother liquor is pumped into the second mother liquor tank by the third transfer pump, and can be discharged through the eighth transfer pump to balance impurities.

[0026] Furthermore, the discharged mother liquor is mixed with the discharged liquid from the first mother liquor tank for further treatment.

[0027] 3. Third-stage crystallization: The mother liquor in the fourth mother liquor tank is pumped into the dissolving vessel by the tenth transfer pump.

[0028] Furthermore, the temperature of the dissolving vessel is controlled at 60~100℃ and maintained for 1~6 hours. After dissolution, the solution is transferred to the crystallization vessel.

[0029] Furthermore, the crystallization temperature is controlled at 5~55℃ and maintained for 1~6h. This temperature range allows for effective separation of the target crystal from impurities. The wet solid is then separated by centrifugation and returned to the dissolving vessel.

[0030] Furthermore, the separated wet solids are transferred to the dissolving vessel by a conveyor belt; the mother liquor is pumped into the third mother liquor tank by a fourth transfer pump.

[0031] 4. Fourth-stage crystallization: The mother liquor in the fifth mother liquor tank is pumped into the dissolving vessel via the eleventh transfer pump.

[0032] Furthermore, the temperature of the dissolving vessel is controlled at 60~100℃ and maintained for 1~6 hours. After dissolution, the solution is transferred to the crystallization vessel.

[0033] Furthermore, the crystallization temperature is controlled at 5~55℃ and maintained for 1~6h. This temperature range allows for effective separation of the target crystal from impurities. The wet solid is then separated by centrifugation and returned to the dissolving vessel.

[0034] Furthermore, the mother liquor is pumped into the fourth mother liquor tank by the fifth transfer pump.

[0035] 5. Fifth stage crystallization: Add fresh deionized water or distilled water to the dissolving vessel.

[0036] Furthermore, this stage uses fresh water to remove residual impurities to the maximum extent, and the water volume is controlled at a liquid-to-solid ratio of 1:1.5~3 (g:mL); after dissolving the wet solids, crystallization and separation are performed.

[0037] Furthermore, the temperature of the dissolving vessel is controlled at 60~100℃ and maintained for 1~6 hours. After dissolution, the solution is transferred to the crystallization vessel.

[0038] Furthermore, the crystallization temperature is controlled at 5~55℃, and the low temperature environment can improve the precipitation rate of the target crystal; the wet solid is the finished product (Mg≤180ppm, Mn≤40ppm, Ti≤25ppm), which is sent to the drying section; the mother liquor is pumped into the fifth mother liquor tank by the sixth transfer pump to complete the countercurrent circulation.

[0039] The technical effects and advantages of this invention are as follows: 1. High-efficiency solvent utilization: The "initialization + continuous countercurrent" mode is adopted, and the mother liquor is reused in stages, which significantly reduces water consumption compared with traditional processes.

[0040] 2. Significantly reduced impurities: Through multi-stage countercurrent crystallization, the content of Mg, Mn, and Ti impurities is significantly reduced, and the product purity meets the requirements of high-end fields. Strong continuity: The process design is adapted to continuous industrial operation, resulting in high production efficiency.

[0041] 3. Environmental protection and economic benefits: The counter-current circulation mode significantly reduces the amount of mother liquor discharged, thereby lowering the cost of environmental treatment. Attached Figure Description

[0042] Figure 1 This is a process flow diagram of the present invention; In the diagram: 1. Dissolving vessel; 2. Steam inlet; 3. Condensate outlet; 4. Dissolving vessel discharge port; 5. First transfer pump; 6. Crystallizing vessel; 7. Chilled water outlet; 8. Chilled water inlet; 9. Crystallizing vessel discharge port; 10. Centrifuge; 11. Centrifuge mother liquor outlet; 12. Centrifuge solids outlet; 13. Second transfer pump; 14. Third transfer pump; 15. Fourth transfer pump; 16. Fifth transfer pump; 17. Sixth transfer pump; 18. First mother liquor tank; 19. Second mother liquor tank; 20. Third mother liquor tank; 21. Fourth mother liquor tank; 22. Fifth mother liquor tank; 23. Seventh transfer pump; 24. Eighth transfer pump; 25. Ninth transfer pump; 26. Tenth transfer pump; 27. Eleventh transfer pump; 28. Conveyor belt. Detailed Implementation

[0043] This invention provides, for example Figure 1 The process flow diagram shown; I. Equipment Description 1. Dissolving vessel 1: 1000L stainless steel jacketed reactor with anchor mechanical agitator at 200rpm. The jacket is heated by 0.3~0.6MPa saturated steam introduced through steam inlet 2. The condensate is discharged through condensate outlet 3. The dissolved material is discharged through dissolving vessel outlet 4 and transported to crystallizing vessel 6 by first transfer pump 5.

[0044] 2. Crystallization vessel 6: 1000L stainless steel jacketed crystallization vessel, with downward mechanical stirring at a stirring speed of 200rpm. The jacket is cooled by -10~10℃ chilled water introduced through the chilled water inlet 8 and discharged through the chilled water outlet 7. The crystallized material flows by gravity from the crystallization vessel outlet 9 to the centrifuge 10.

[0045] 3. Centrifuge 10: Flat plate centrifuge, with a processing capacity of 200 kg / h and a separation factor of 850; the mother liquor flows by gravity from the centrifuge mother liquor outlet 11 to the mother liquor tank, and the solids flow from the centrifuge solids outlet 12 to the conveyor belt 28. 4. Mother liquor tanks: The first mother liquor tank 18 to the fifth mother liquor tank 22 are all 500L stainless steel storage tanks, equipped with magnetic float level gauges and insulation layers; 5. Transfer pumps: Transfer pumps 1 through 11 (27) are all corrosion-resistant centrifugal pumps with a flow rate of 150-250 L / h and a head of 10-15 m; 6. Conveyor belt 28: made of stainless steel, with a conveying speed of 200 kg / h, used for solid transfer between centrifuge 10 and dissolving vessel.

[0046] II. Process Examples Example 1

[0047] 1. System initialization and initial material processing stage: This stage aims to establish a mother liquor storage system and obtain low-impurity starting wet solids, laying the foundation for continuous production. The crude product incoming material is 280 kg on a dry basis, with initial impurity contents of: Mg 1797 ppm, Mn 506 ppm, and Ti 38.5 ppm.

[0048] (1) First crystallization: 280 kg of crude product was added to dissolving vessel 1, and 420 L of fresh deionized water was added (mass-volume ratio of approximately 1:1.5).

[0049] Heat the dissolving vessel 1 to 85°C and stir at 200 rpm for 2 hours to ensure the material is completely dissolved.

[0050] After dissolving, the solution is pumped into crystallizer 6 via the first transfer pump, cooled to 15°C by introducing chilled water, and crystallized for 4 hours with a stirring speed of 200 rpm.

[0051] After crystallization, the material is separated in centrifuge 10 for 30 minutes, and the moisture content of the wet solids is ≤10%.

[0052] The wet solids (Mg 552 ppm, Mn 138 ppm, Ti 28.5 ppm) are returned to the dissolving vessel 1 via conveyor belt 28, and the mother liquor is pumped into the first mother liquor tank 18 via the second transfer pump 13.

[0053] (2) Secondary crystallization: Add 480 L of fresh deionized water to dissolve the returned wet solids.

[0054] Heat to 85℃, stir and dissolve for 2 hours, then transfer to crystallization vessel 6, cool to 15℃ and crystallize for 4 hours.

[0055] After centrifugation, the wet solids are returned to dissolving vessel 1, and the mother liquor is pumped into the second mother liquor tank 19.

[0056] (3) Tertiary crystallization: Add 420 L of fresh deionized water and proceed as with secondary crystallization.

[0057] After centrifugation, the wet solids are returned to dissolving vessel 1, and the mother liquor is pumped into the third mother liquor tank 20.

[0058] (4) Quadruple crystallization: Add 400 L of fresh deionized water and proceed as before.

[0059] After centrifugation, the wet solids are returned to dissolving vessel 1, and the mother liquor is pumped into the fourth mother liquor tank 21.

[0060] (5) Five crystallizations: Add 390 L of fresh deionized water and proceed as before.

[0061] After centrifugation, the initial wet solid was obtained, with impurity contents of Mg 175 ppm, Mn 38 ppm, and Ti 24 ppm.

[0062] 2. Continuous Countercurrent Crystallization Production Stage This stage involves continuous production based on the initialized system. The new crude product, on a dry basis, weighs 280 kg, and its impurity content is the same as in the initialization stage. Each crystallization stage is controlled with a dissolution temperature of 85℃, a crystallization temperature of 15℃, a crystallization time of 4 h, and a stirring speed of 200 rpm.

[0063] (1) First-order crystallization: 280 kg of the new crude product was added to dissolving vessel 1, and 420 L of fresh deionized water was added (mass-volume ratio 1:1.5).

[0064] After dissolving, the solution is transferred to crystallization vessel 6 for crystallization and then separated by centrifugation.

[0065] The wet solids are returned to the dissolving vessel 1, and the mother liquor is pumped into the first mother liquor tank 18 (part of the mother liquor can be discharged through the seventh transfer pump 23 to control impurities).

[0066] (2) Secondary crystallization: The mother liquor 420 L in the third mother liquor tank 20 is pumped into the dissolving vessel 1 through the ninth transfer pump 25 to dissolve the returned wet solids.

[0067] After crystallization and centrifugation, the wet solids are returned to the dissolving vessel 1, and the mother liquor is pumped into the second mother liquor tank 19 (part of the mother liquor can be discharged through the eighth transfer pump 24).

[0068] (3) Tertiary crystallization: The 400 L of mother liquor in the fourth mother liquor tank 21 is pumped into the dissolving vessel 1 through the tenth transfer pump 26 to dissolve the returned wet solids.

[0069] After crystallization and centrifugation, the wet solid is returned to dissolving vessel 1, and the mother liquor is pumped into the third mother liquor tank 20.

[0070] (4) Quaternary crystallization: The 390 L of mother liquor in the fifth mother liquor tank 22 is pumped into the dissolving vessel 1 through the eleventh transfer pump 27 to dissolve the returned wet solids.

[0071] After crystallization and centrifugation, the wet solid is returned to dissolving vessel 1, and the mother liquor is pumped into the fourth mother liquor tank 21.

[0072] (5) Fifth-order crystallization: Add 400 L of fresh deionized water (mass-volume ratio of approximately 1:1.43) to dissolve the returned wet solids.

[0073] After crystallization and centrifugation, the wet solid is the finished product and is sent to the drying section (vacuum drying at 50℃ for 2 hours, with a moisture content of ≤0.5%).

[0074] The mother liquor is pumped into the fifth mother liquor tank 22.

[0075] Results: The impurity content of the finished product was 175 ppm Mg, 38 ppm Mn, and 24 ppm Ti, and the solvent water consumption was reduced by 42% compared with the traditional process. Example 2

[0076] 1. System initialization and initial material processing stage The crude product arrived at a dry basis of 230 kg, with initial impurity content of Mg 1820 ppm, Mn 520 ppm, and Ti 40 ppm.

[0077] (1) First crystallization: 230 kg of crude product was added to dissolving vessel 1, and 345 L of fresh deionized water was added (mass-volume ratio 1:1.5).

[0078] The dissolution temperature was 85℃, the crystallization temperature was 20℃, and the crystallization time was 3 h.

[0079] After centrifugation, the wet solids are returned to dissolving vessel 1, and the mother liquor is stored in the first mother liquor tank 18.

[0080] (2) Second to fifth crystallization: Add 450 L, 420 L, 400 L and 400 L of fresh deionized water respectively, and repeat the dissolution-crystallization-separation operation.

[0081] The final starting wet solids were obtained with the following impurity contents: Mg 168 ppm, Mn 35 ppm, and Ti 23 ppm.

[0082] 2. Continuous Countercurrent Crystallization Production Stage 230 kg of the new crude product (dry basis) was used. For each crystallization stage, the dissolution temperature was controlled at 85℃, the crystallization temperature at 20℃, the crystallization time at 3 h, and the stirring speed at 250 rpm.

[0083] (1) First stage crystallization: Add 345 L of fresh deionized water to the new crude product, dissolve and crystallize, separate, return the wet solid, and put the mother liquor into the first mother liquor tank 18.

[0084] (2) Second stage crystallization: Dissolve wet solids using 420 L of mother liquor from the third mother liquor tank 20. After crystallization, transfer the mother liquor to the second mother liquor tank 19.

[0085] (3) Third stage crystallization: Dissolve wet solids with 400 L of mother liquor in the fourth mother liquor tank 21. After crystallization, the mother liquor is transferred to the third mother liquor tank 20.

[0086] (4) Fourth stage crystallization: Dissolve the wet solid with 400 L of mother liquor from the fifth mother liquor tank 22. After crystallization, the mother liquor is transferred to the fourth mother liquor tank 21.

[0087] (5) Fifth stage crystallization: Add 400 L of fresh deionized water. The wet solid after crystallization is the finished product and is sent to the drying section.

[0088] Results: The impurity content of the finished product was 168 ppm Mg, 35 ppm Mn, and 23 ppm Ti, and the solvent water consumption was reduced by 45% compared with the traditional process. Example 3

[0089] 1. System initialization and initial material processing stage The crude product arrived at a dry basis of 230 kg, with initial impurity content of Mg 1800 ppm, Mn 510 ppm, and Ti 39 ppm.

[0090] (1) First crystallization: 230 kg of crude product was added to dissolving vessel 1, and 345 L of fresh deionized water was added (mass-volume ratio 1:1.5).

[0091] The dissolution temperature was 75℃, the crystallization temperature was 18℃, and the crystallization time was 5 h.

[0092] After centrifugation, the wet solids are returned to dissolving vessel 1, and the mother liquor is stored in the first mother liquor tank 18.

[0093] (2) Second to fifth crystallization: Add 430 L, 420 L, 420 L and 420 L of fresh deionized water respectively, and repeat the operation.

[0094] The final starting wet solids were obtained with the following impurity contents: Mg 172 ppm, Mn 36 ppm, and Ti 22 ppm.

[0095] 2. Continuous Countercurrent Crystallization Production Stage 230 kg of the new crude product (dry basis) was used. For each crystallization stage, the dissolution temperature was controlled at 75℃, the crystallization temperature at 18℃, the crystallization time at 5 h, and the stirring speed at 150 rpm.

[0096] (1) First stage crystallization: Add 345 L of fresh deionized water to the new crude product, dissolve and crystallize, separate, return the wet solid, and put the mother liquor into the first mother liquor tank 18.

[0097] (2) Second stage crystallization: Dissolve wet solids using 420 L of mother liquor from the third mother liquor tank 20. After crystallization, transfer the mother liquor to the second mother liquor tank 19.

[0098] (3) Third-stage crystallization: Dissolve the wet solids in 420 L of mother liquor in the fourth mother liquor tank 21. After crystallization, the mother liquor is transferred to the third mother liquor tank 20.

[0099] (4) Fourth stage crystallization: Dissolve the wet solid with 420 L of mother liquor from the fifth mother liquor tank 22. After crystallization, the mother liquor is transferred to the fourth mother liquor tank 21.

[0100] (5) Fifth stage crystallization: Add 420 L of fresh deionized water. The wet solid after crystallization is the finished product and is sent to the drying section.

[0101] Results: The impurity content of the finished product was 172 ppm Mg, 36 ppm Mn, and 22 ppm Ti, and the solvent water consumption was reduced by 43% compared with the traditional process.

[0102] The above embodiments demonstrate that the process of the present invention can stably control the Mg, Mn, and Ti impurities in ferrous sulfate heptahydrate within the target range, and has the advantages of low solvent consumption, low mother liquor discharge, and continuous production, making it fully suitable for the needs of large-scale industrial production.

[0103] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A countercurrent crystallization process for reducing the impurity content in ferrous sulfate heptahydrate, characterized in that, The process includes system initialization and initial material handling, as well as continuous countercurrent crystallization production. The specific steps are as follows: Step 1: System Initialization and Initial Material Handling Stage ① First crystallization: The crude product is fed into the dissolving kettle, heated to dissolve, and then pumped into the crystallization kettle to cool and crystallize. After separation by centrifuge, the wet solid is returned to the dissolving kettle, and the mother liquor is pumped into the first mother liquor tank. ② Second to fifth crystallization: During the nth crystallization (n=2,3,4,5), fresh solvent water is added to the dissolving vessel to dissolve the returned wet solid. After crystallization in the crystallizing vessel and separation by centrifuge, the mother liquor is pumped into the nth mother liquor tank, and the wet solid is returned to the dissolving vessel, finally obtaining the low-impurity starting wet solid. Step 2: Continuous countercurrent crystallization production stage ① First-stage crystallization: The new crude product is put into the dissolving kettle, and after dissolving, it is pumped into the crystallization kettle for crystallization. After separation by centrifuge, the wet solid is returned to the dissolving kettle, and the mother liquor is pumped into the first mother liquor tank; ② Second-stage crystallization: The mother liquor in the third mother liquor tank is pumped into the dissolving kettle to dissolve the wet solid. After crystallization and separation, the wet solid is returned to the dissolving kettle, and the mother liquor is pumped into the second mother liquor tank. ③ Third-stage crystallization: The mother liquor in the fourth mother liquor tank is pumped into the dissolving kettle to dissolve the wet solid. After crystallization and separation, the wet solid is returned to the dissolving kettle, and the mother liquor is pumped into the third mother liquor tank. ④ Fourth stage crystallization: The mother liquor in the fifth mother liquor tank is pumped into the dissolving kettle to dissolve the wet solid. After crystallization and separation, the wet solid is returned to the dissolving kettle, and the mother liquor is pumped into the fourth mother liquor tank. ⑤ Fifth stage crystallization: Fresh solvent water is added to the dissolving vessel to dissolve the wet solid. After crystallization and separation, the wet solid is sent to the drying section as the finished product, and the mother liquor is pumped into the fifth mother liquor tank.

2. The process according to claim 1, characterized in that, The dissolving vessel is equipped with an anchor-type mechanical stirrer with a stirring speed of 30~500 rpm; the crystallizing vessel is equipped with a pressure-type mechanical stirrer with a stirring speed of 30~500 rpm.

3. The process according to claim 1, characterized in that, The centrifuge is a flat-plate centrifuge or a continuous feed centrifuge, with a processing capacity of 5~15000 kg / h.

4. The process according to claim 1, characterized in that, The first to fifth mother liquor tanks are all stainless steel or plastic-lined tanks, equipped with level gauges and inlet / outlet pump interfaces.

5. The process according to claim 1, characterized in that, The dissolving temperature in the dissolving vessel is 60~100℃, and the crystallization temperature in the crystallization vessel is 5~55℃.

6. The process according to claim 1, characterized in that, The mass-to-volume ratio of the crude product dry basis to the solvent water is 1:1.5 to 1:

10.

7. The process according to claim 1, characterized in that, During the continuous countercurrent crystallization production stage, the mother liquor in the first mother liquor tank and / or the second mother liquor tank can be partially discharged to control the system impurity balance.

8. The process according to claim 1, characterized in that, The first to eleventh transfer pumps are all corrosion-resistant centrifugal pumps with a flow rate of 0.05~30m³ / h and a head of 1~50m.

9. The process according to claim 1, characterized in that, The preferred crystallization temperature for the initial crystallization and continuous countercurrent crystallization production stages is 15℃, and the preferred crystallization time is 4h; the preferred dissolution temperature is 85℃.

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