Multi-stage continuous crystallization method of high-purity large-granularity ellipsoidal baking soda crystals

Abstract

The invention provides a multi-stage continuous crystallization method of high-purity large-granularity ellipsoidal baking soda crystals. The continuous crystallization comprises a reaction crystallization process, a first-stage cooling crystallization process and a second-stage cooling crystallization process which are continuously arranged; compared with intermittent crystallization, the continuous crystallization can improve the production efficiency of baking soda crystals, reduce the difference between batches and reduce the energy consumption; in addition, the baking soda crystal prepared by the continuous crystallization method is good in morphology and is in a monodisperse block shape; the particle size is uniform, and the average particle size is 300-450 microns; the flowability of the product is good, and the angle of repose is 25-35 degrees; the purity of the product is higher than 99.5%, impurities are few, and the quality standard of Chinese pharmacopoeia is met.

Description

Technical Field

[0001] The invention belongs to the field of inorganic salt crystallization, and relates to a multi-stage continuous crystallization method of high-purity, large-size, ellipsoidal baking soda crystals. Background Art

[0002] Sodium bicarbonate, commonly known as baking soda, is an important basic chemical raw material and commonly used medicine. Its application fields span the chemical, pharmaceutical, food, light industry and textile industries. In the medical field, baking soda is one of the earliest easily absorbed antacids. Due to its precise efficacy and low price, it is still a "veteran" drug widely used around the world. As countries have higher and higher requirements for raw materials, the market demand for high-quality baking soda continues to increase.

[0003] In the field of hemodialysis, baking soda is one of the main raw materials for dialysis powder. At present, most of the baking soda products produced in China are food grade and industrial grade, and very few can meet the standards of medicinal baking soda. In addition, most of the baking soda produced in China is powdered, with small particle size (main particle size below 200 microns), easy to agglomerate when dissolved, and poor fluidity, which is not only not conducive to the rapid dissolution of baking soda, but also very easy to block the catheter during the use of the hemodialysis machine, delay the treatment time, and affect the adequacy and safety of the hemodialysis treatment process. Therefore, at present, the baking soda products that can be directly used in the dialysis machine used by most hemodialysis centers in my country are large-particle baking soda imported from Solvay. However, the high cost of imported dialysis products has increased the economic burden on patients, so the development of large-particle (main particle size greater than 300 microns) and high-purity baking soda products is an urgent problem to be solved in my country's baking soda industry.

[0004] For the preparation of baking soda by double decomposition reaction, there is a common disadvantage that the growth of baking soda crystal particles is difficult to control. CN101185484B and CN101696022B disclose a production process for continuously preparing food-grade baking soda by double decomposition reaction of brine and ammonium bicarbonate. Due to the purity of brine and ammonium bicarbonate raw materials, the purity of baking soda products reaches non-pharmaceutical grade, and ammonium carbonate is solid, and ammonium carbonate is easily wrapped in solid-liquid reaction, and the product has a high ammonium content and a small particle size. CN110282638A discloses a double decomposition process by adding an ammonium bicarbonate dissolving tank, in which ammonium bicarbonate is first dissolved and then mixed with brine. This process optimizes the growth environment of the product sodium bicarbonate crystals, reduces peritectic (solid sodium bicarbonate wraps solid ammonium bicarbonate), and reduces the cost of washing and post-treatment. However, the dissolution of ammonium bicarbonate requires a large amount of water, which increases the volume of the reaction solution per unit product (producing the same amount of baking soda), greatly increases the equipment cost, and results in the generation of a large amount of waste water. In addition, the double decomposition reaction temperature is relatively low, and low temperature is not conducive to the growth of crystals. The resulting product has a small particle size, and the main particle size of the product will be less than 200 microns.

[0005] CN111634928A discloses a method for preparing large-particle baking soda by secondary carbonization and the resulting baking soda. The raw material liquid of the invention undergoes a pre-carbonization process in a packing tower and a secondary carbonization process in a carbonization tower. This process requires the addition of additives such as sodium polystyrene sulfonate, which is difficult to meet medical grade use. The main particle size of the product is 180 to 320 microns, the product particle size is small and the batch fluctuation is large. The product is mainly in the form of flakes or long needles, and the powder properties are poor.

[0006] CN108996526A discloses a method for preparing large-particle heavy baking soda, by adding additives such as calcium chloride, sodium hexametaphosphate or DL-tartaric acid to obtain large-particle heavy baking soda, the main particle size of the product is 180-290 microns, and the product morphology is a flaky product. CN103172090A discloses a method for producing industrial-grade baking soda. CN115650259B discloses a method and device for preparing large-particle sodium bicarbonate, by carbonization reaction, cooling crystallization and fine crystal elimination to obtain a large-particle product, the particle size is mainly 180-425 microns, but the product is an agglomerate, and a fine crystal elimination device and operation are also required, and the equipment and operation costs are high.

[0007] Therefore, there is an urgent need for an efficient processing method for preparing high-purity, large-particle ellipsoidal continuous crystallization of sodium thiocarbamide, which can not only improve the quality of the crystal product, but also improve the efficiency of the crystallization process and reduce production costs. Summary of the invention

[0008] The invention aims to provide a multi-stage continuous crystallization method capable of preparing high-purity, large-size, ellipsoidal sodium bicarbonate crystals. The obtained sodium bicarbonate crystals have good morphology and are monodisperse ellipsoidal; the particle size is highly uniform and the average particle size is 350-450 microns; the product has high fluidity and the angle of repose is between 25° and 35°; the product has a purity higher than 99.5% and few impurities, and meets the quality standards of the Chinese Pharmacopoeia.

[0009] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions: One of the purposes of the present invention is to provide a multi-stage continuous crystallization method for high-purity large-particle ellipsoidal baking soda, the multi-stage continuous crystallization method comprising the following steps: (1) Under stirring conditions, the temperature of the reaction crystallizer is kept constant at 75-85°C (for example, 75°C, 78°C, 80°C, 82°C, 85°C, etc.), sodium carbonate solution is continuously added to the reaction crystallizer, and 70-90% (for example, 70%, 75%, 80%, 85%, 90%, etc.) of the total carbon dioxide feed rate is continuously introduced into the reaction crystallizer; wherein, a crystal slurry with a crystal particle size of 100-300 μm (for example, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, etc.) is discharged from the reaction crystallizer and transported to a primary cooling crystallizer, and a crystal slurry with a crystal particle size of less than 100 μm is returned to the reaction crystallizer; (2) Under stirring conditions, 5-15% (e.g., 5%, 7%, 10%, 12%, 15%, etc.) of the total feed rate of carbon dioxide is continuously added to the primary cooling crystallizer, and the primary cooling crystallizer reduces the temperature of the feed liquid to 65-75°C (e.g., 65°C, 68°C, 70°C, 72°C, 75°C, etc.) for cooling crystallization. The slurry is discharged from the primary cooling crystallizer and transported to the secondary cooling crystallizer; (3) Under stirring conditions, 5-15% (e.g., 5%, 7%, 10%, 12%, 15%, etc.) of the total feed rate of carbon dioxide is continuously added to the secondary cooling crystallizer, and the secondary cooling crystallizer reduces the temperature of the feed liquid to 50-65°C (e.g., 50°C, 55°C, 60°C, 65°C, etc.) for cooling crystallization. The slurry is discharged from the secondary cooling crystallizer and enters the solid-liquid separation system; (4) In the solid-liquid separation system, the separated solid is discharged from the solid-liquid separation system, and the mother liquor enters the sodium carbonate dissolving tank to dissolve the sodium carbonate, and then enters the reaction crystallizer for recycling.

[0010] It should be noted that during the cooling process of the primary cooling crystallizer and the secondary cooling crystallizer, the temperature of the feed liquid in the primary cooling crystallizer is lower than that in the reaction crystallizer, and the temperature of the feed liquid in the secondary cooling crystallizer is lower than that in the primary cooling crystallizer.

[0011] Preferably, the mass ratio of sodium carbonate to water in the sodium carbonate solution in step (1) is 1:3-8, for example 1:8, 2:8, 3:8, etc.

[0012] Preferably, the sodium carbonate solution in step (1) is fed in 1 to 3 streams; The reaction crystallizer in step (1) comprises a main structure, a stirring device and a circulation system; It should be noted that this application does not specifically limit the main structure of the reaction crystallizer, and those skilled in the art can make adjustments according to actual needs. The function of the stirring system is to partition the slurry and form different flow paths; the circulation device includes a slurry circulation system and a carbon dioxide circulation system. The slurry circulation system is to circulate the small particle crystals in the slurry internally to promote the growth of small particles into large particle crystals, and the carbon dioxide circulation system is to inject the unreacted carbon dioxide back into the reaction crystallizer for reaction to increase the reaction rate of carbon dioxide, thereby reducing the waste of carbon dioxide; this application does not specifically limit the specific structure of the circulation system, and those skilled in the art can make adjustments according to actual needs.

[0013] Preferably, the stirring device comprises multiple layers of stirring paddles, preferably 2-10 layers of stirring paddles, and more preferably 3 layers of stirring paddles; Preferably, the circulation system includes a carbon dioxide circulation system and a slurry circulation system.

[0014] Preferably, when the stirring device is a three-layer stirring paddle, the stirring device includes a bottom stirring paddle, a middle stirring paddle and an upper stirring paddle from bottom to top; Preferably, the bottom stirring paddle is a six-blade disc stirring paddle, and the middle stirring paddle and the upper stirring paddle are both four-blade downward pressure propulsion stirring paddles; Preferably, the area below the bottom stirring paddle and between the bottom stirring paddle and the middle stirring paddle are both nucleation areas; Preferably, the area above the upper stirring paddle and between the upper stirring paddle and the middle stirring paddle are both growth areas.

[0015] Preferably, the total carbon dioxide feed rate in step (1) is 2%-150% / min of the effective volume of the reaction crystallizer, for example 2% / min, 10% / min, 2% / min, 30% / min, 40% / min, 50% / min, 60% / min, 70% / min, 80% / min, 90% / min, 100% / min, 110% / min, 120% / min, 130% / min, 140% / min, 150% / min, etc.

[0016] Preferably, the carbon dioxide in step (1) is divided into 1-4 gas feeds; Preferably, the tail gas formed after the carbon dioxide reacts in step (1) is compressed and then re-enters the reaction crystallizer for recycling.

[0017] Preferably, in step (2), the rate at which the slurry enters the primary cooling crystallizer is the same as the rate at which it exits the primary cooling crystallizer; Preferably, the primary cooling crystallizer in step (2) comprises a main structure and a stirring device arranged in the main structure, and a guide tube is provided on the outside of the stirring device.

[0018] The present application does not specifically limit the main structure of the primary cooling crystallizer, and those skilled in the art may make adjustments according to actual needs.

[0019] Preferably, in step (3), the rate at which the slurry enters the secondary cooling crystallizer is the same as the rate at which it exits the secondary cooling crystallizer; Preferably, the secondary cooling crystallizer in step (3) comprises a main structure and a stirring device arranged in the main structure, and a guide tube is provided on the outside of the stirring device.

[0020] The present application does not specifically limit the main structure of the secondary cooling crystallizer, and those skilled in the art may make adjustments according to actual needs.

[0021] Preferably, step (4) further comprises washing and drying the separated solids in sequence; Preferably, the cleaning solvent is a saturated baking soda solution, and the cleaning times are 1-3 times.

[0022] The second object of the present invention is to prepare baking soda crystals according to the multi-stage continuous crystallization method described in the first object; Preferably, the baking soda product has a purity of more than 99.5%, the main particle size of the product is distributed in the range of 300-450 microns (for example, 300 microns, 320 microns, 350 microns, 370 microns, 400 microns, 420 microns, 450 microns), the crystals are monodisperse ellipsoidal morphology, the product has good fluidity, and the angle of repose is 25-35° (for example, 25°, 27°, 30°, 32°, 35°, etc.).

[0023] The third object of the present invention is to use the large-particle baking soda crystals described in the second object in the pharmaceutical industry.

[0024] Compared with the prior art, the present invention has the following beneficial effects: The baking soda crystals prepared by the continuous crystallization method of the present invention have good morphology and are monodisperse ellipsoids; the particle size is highly uniform, and the average particle size is 350 to 450 microns; the product has high fluidity, and the angle of repose is between 25° and 35°; the product purity is higher than 99.5%, and the impurities are small, which meets the quality standards of the Chinese Pharmacopoeia. The equipment avoids the use of a carbonization tower and a fine crystal elimination tank, and the gas-liquid reaction and the crystallization process occur simultaneously in the reaction crystallizer, the operation and control are simple, and the equipment utilization rate and production efficiency are high. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1is a schematic diagram of the continuous crystallization device in Example 1; Among them, 1 is a reaction crystallizer; 2 is a primary cooling crystallizer; 3 is a secondary cooling crystallizer.

[0026] Figure 2 is a stereomicroscope image of the product obtained in Example 1, with a scale of 500 μm; Figure 3 This is the particle size distribution diagram of the product obtained in Example 1; Figure 4 is a scanning electron microscope image of the product obtained in Example 2, with a scale of 300 μm; Figure 5 is a scanning electron microscope image of the product obtained in Example 3, with a scale of 300 μm; Figure 6 This is a scanning electron microscope image of a commercially available product, with a scale of 2 mm. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0028] Structure example This embodiment provides a continuous crystallization device for high-purity large-size ellipsoidal sodium bicarbonate crystals, such as Figure 1 As shown, it includes a reaction crystallization kettle, a primary cooling crystallizer and a secondary cooling crystallizer connected in series; wherein the reaction crystallization kettle includes a main structure and a stirring device arranged in the main structure, wherein the bottom stirring paddle is a six-blade disc stirring paddle, and the middle stirring paddle and the upper stirring paddle are both four-blade downward pressure propulsion stirring paddles; the structure of the primary cooling crystallizer is a crystallizer with a guide tube, and the structure of the secondary cooling crystallizer is a crystallizer with a guide tube; wherein the reaction crystallizer, the primary cooling crystallizer and the secondary cooling crystallizer are connected by pipelines, and the slurry is transported through the pipeline during the transportation process and is transported under the action of a pump.

[0029] Since the continuous crystallization production operation includes the initial start-up stage and the continuous operation stage of stable operation, in order to facilitate those skilled in the art to better understand this patent, the following embodiments include the initial start-up operation and the continuous operation process of stable operation.

[0030] Example 1 This embodiment also provides a continuous crystallization method for high-purity large-size ellipsoidal baking soda crystals, comprising the following steps: Initial start-up operation: (1) At a temperature of 80°C, purified water, sodium carbonate and sodium bicarbonate at a temperature of 80°C were added to the reaction crystallizer, the primary cooling crystallizer and the secondary cooling crystallizer respectively under stirring conditions to prepare a solution with a sodium carbonate concentration of 15wt% and a sodium bicarbonate concentration of 10wt%; then sodium bicarbonate seeds with an average particle size of 300 μm were added (after the addition of the seeds, the solid-liquid mass ratio in the solution was 5wt%), and the crystals were grown for 20 minutes to prepare an initial base solution; (2) Under stirring conditions, the primary cooling crystallizer was cooled to 70°C at a cooling rate of 2.5°C / h; the secondary cooling crystallizer was cooled to 60°C at a cooling rate of 2.5°C / h.

[0031] Continuous run operation: (1) Under stirring conditions, the temperature of the reaction crystallization kettle is controlled to be 80°C, and a sodium carbonate solution with a concentration of 20 g / L at 80°C is continuously added. The sodium carbonate solution is added at a rate of 0.3 times the effective volume of the reaction crystallizer per hour. 80% of the total feed rate of carbon dioxide gas (based on the effective volume of the reaction crystallizer being 1 L, the total feed rate of carbon dioxide is 1 L / min) is continuously introduced into the reaction crystallization kettle. Crystal slurry with a crystal particle size of 100-300 μm is continuously discharged from the reaction crystallization kettle and enters a primary cooling crystallizer. Crystal slurry with a crystal particle size of less than 100 μm is returned to the upper growth zone of the reaction crystallizer. The liquid level in the reaction crystallizer is maintained constant. (2) Under stirring conditions, the temperature of the primary cooling crystallizer is controlled at 70°C, 10% of the total feed rate of carbon dioxide is continuously added into the primary cooling crystallizer, the slurry is continuously discharged from the bottom of the crystallizer and enters the secondary cooling crystallizer, and the liquid level in the primary cooling crystallizer is maintained constant; (3) Under stirring conditions, the temperature of the secondary cooling crystallizer is controlled to be 60°C, and 10% of the total feed rate of carbon dioxide is continuously added into the secondary cooling crystallizer. The slurry is discharged from the secondary cooling crystallizer and enters the solid-liquid separation system; (4) In the solid-liquid separation system, the separated solid is washed three times with a saturated sodium bicarbonate solution, and the product is filtered and dried to obtain baking soda crystals; the separated liquid is circulated to the sodium carbonate dissolving tank to dissolve the sodium carbonate, thereby realizing the recycling of the mother body.

[0032] Figure 2 This is a stereomicroscope image of the product obtained in Example 1. Figure 3 The particle size distribution diagram of the product obtained in Example 1 shows that the particle size distribution is 300-450 microns, and the average particle size is 352 μm.

[0033] The product obtained in Example 1 was tested for purity by titration and found to have a purity of 99.8%.

[0034] The product obtained in Example 1 was subjected to an angle of repose test, and the angle of repose was found to be 29°.

[0035] Figure 6 This is a scanning electron microscope image of a commercially available product. Figure 2 and Figure 6 By comparison, it can be clearly seen that the particle size of the product obtained in Example 1 is superior to that of the commercially available product.

[0036] Example 2 Initial start-up operation: (1) At a temperature of 75°C, purified water, sodium carbonate and sodium bicarbonate at a temperature of 75°C were added to the reaction crystallizer, the primary cooling crystallizer and the secondary cooling crystallizer respectively under stirring conditions to prepare a solution with a sodium carbonate concentration of 15wt% and a sodium bicarbonate concentration of 9.5wt%, and then sodium bicarbonate seeds with an average particle size of 200 μm were added (after the seeds were added, the solid-liquid mass ratio in the solution was 8wt%), and the crystals were grown for 10 minutes to prepare an initial base solution; (2) Under stirring conditions, the primary cooling crystallizer was cooled to 65°C at a cooling rate of 2°C / h; the secondary cooling crystallizer was cooled to 60°C at a cooling rate of 2°C / h.

[0037] Continuous run operation: (1) Under stirring conditions, the temperature of the reaction crystallizer is controlled at 75°C, and a sodium carbonate solution with a concentration of 20 g / L at 75°C is continuously added. The sodium carbonate solution is added at a rate of 0.3 times the effective volume of the reaction crystallizer per hour. Then, 70% of the total feed rate of carbon dioxide gas (based on the volume of the reaction crystallizer being 1 L, the total feed rate of carbon dioxide is 1 L / min) is continuously introduced into the reaction crystallizer. Crystal slurry with a crystal size of 100-300 μm is continuously discharged from the reaction crystallizer and enters a primary cooling crystallizer. Crystal slurry with a crystal size of less than 100 μm is returned to the upper growth zone of the reaction crystallizer. The liquid level in the reaction crystallizer is maintained constant. (2) Under stirring conditions, 15% of the total carbon dioxide feed rate is continuously added to the primary cooling kettle, and the slurry is continuously discharged from the bottom of the crystallizer and enters the secondary cooling crystallizer. The liquid level in the primary cooling crystallizer is maintained constant; (3) Under stirring conditions, 15% of the total feed rate of carbon dioxide is continuously added into the secondary cooling kettle, and the slurry is discharged from the secondary cooling crystallizer and enters the solid-liquid separation system, and the liquid level in the secondary cooling crystallizer is maintained constant; (4) In the solid-liquid separation system, the separated solid is washed three times with a saturated sodium bicarbonate solution, filtered and dried to obtain baking soda crystals; the separated liquid is circulated to the reaction crystallizer for use.

[0038] Figure 4The scanning electron microscope image of the product obtained in Example 2 shows that the particle size distribution is 300-450 microns. The baking soda crystals obtained in Example 2 were tested for particle size distribution, purity, and fluidity, and it was found that the average main particle size of the product was 342 μm, the purity was 99.7%, and the angle of repose was 32°.

[0039] Example 3 Initial start-up operation: (1) At a temperature of 85°C, purified water, sodium carbonate and sodium bicarbonate at a temperature of 85°C were added to the reaction crystallizer, the primary cooling crystallizer and the secondary cooling crystallizer respectively under stirring conditions to prepare a solution with a sodium carbonate concentration of 15wt% and a sodium bicarbonate concentration of 10wt%, and then sodium bicarbonate seeds with an average particle size of 150 μm were added (after the addition of the seeds, the solid-liquid mass ratio in the solution was 2wt%), and the crystals were grown for 30 minutes to prepare an initial bottom solution; (2) Under stirring conditions, the primary cooling crystallizer was cooled to 75°C at a cooling rate of 3°C / h; the secondary cooling crystallizer was cooled to 70°C at a cooling rate of 3°C / h.

[0040] Continuous run operation: (1) Under stirring conditions, the temperature of the reaction crystallizer is controlled to be 85°C, and a sodium carbonate solution of 85°C is continuously added, and then 90% of the total feed rate of carbon dioxide gas (based on the volume of the reaction crystallizer being 1 L, the total feed rate of carbon dioxide is 1 L / min) is continuously introduced into the reaction crystallizer, and the crystal slurry with a crystal particle size of 100-300 μm is continuously discharged from the reaction crystallizer and enters the primary cooling crystallizer, and the crystal slurry with a crystal particle size of less than 100 μm is returned to the upper growth zone of the reaction crystallizer, and the reaction crystallizer maintains a constant liquid level; (2) Under stirring conditions, 5% of the total carbon dioxide feed rate is continuously added to the primary cooling crystallizer, and the slurry is continuously discharged from the bottom of the crystallizer and enters the secondary cooling crystallizer. The liquid level in the primary cooling crystallizer is maintained constant; (4) Under stirring conditions, 5% of the total feed rate of carbon dioxide is continuously added into the secondary cooling crystallizer, and the slurry is discharged from the bottom of the secondary cooling crystallizer and enters the solid-liquid separation system. The liquid level in the secondary cooling crystallizer is maintained constant; (5) In the solid-liquid separation system, the separated solid is washed three times with a saturated sodium bicarbonate solution, filtered and dried to obtain baking soda crystals; the separated liquid is circulated to the reaction crystallizer for use.

[0041] Figure 5The scanning electron microscope image of the product obtained in Example 3 shows that the particle size distribution is 300-450 microns. The baking soda crystals obtained in Example 3 were tested for particle size distribution, purity, and fluidity. It was found that the average main particle size of the product was 348 μm, the purity was 99.9%, and the angle of repose was 29°.

[0042] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A multi-stage continuous crystallization method for high-purity large-size ellipsoidal sodium bicarbonate crystals, characterized in that: The multi-stage continuous crystallization method comprises the following steps: (1) Under stirring conditions, the temperature of the reaction crystallization kettle is kept constant at 75-85° C., sodium carbonate solution is continuously added to the reaction crystallization kettle, and 70-90% of the total feed rate of carbon dioxide is continuously passed into the reaction crystallization kettle; Among them, the crystal slurry with a crystal particle size of 100-300 μm is discharged from the reaction crystallizer and transported to the primary cooling crystallizer, and the crystal slurry with a crystal particle size less than 100 μm returns to the reaction crystallizer; (2) Under stirring conditions, 5-15% of the total feed rate of carbon dioxide is continuously added to the primary cooling crystallizer. The primary cooling crystallizer reduces the temperature of the feed liquid to 65-75°C for cooling crystallization. The slurry is discharged from the primary cooling crystallizer and transported to the secondary cooling crystallizer. (3) Under stirring conditions, 5-15% of the total feed rate of carbon dioxide is continuously added to the secondary cooling crystallizer, and the secondary cooling crystallizer reduces the temperature of the feed liquid to 50-65°C for secondary cooling crystallization. The slurry is discharged from the secondary cooling crystallizer and enters the solid-liquid separation system; (4) In the solid-liquid separation system, the separated solid is discharged from the solid-liquid separation system, and the mother liquor enters the sodium carbonate dissolving tank to dissolve the sodium carbonate, and then enters the reaction crystallizer for recycling.

2. The multi-stage continuous crystallization method according to claim 1, characterized in that: The mass ratio of sodium carbonate to water in the sodium carbonate solution of step (1) is 1:3-8; Preferably, the sodium carbonate solution in step (1) is fed in 1 to 3 streams.

3. The preparation method according to claim 1, characterized in that: The reaction crystallization kettle in step (1) comprises a main structure, a stirring device and a circulation system; Preferably, the stirring device comprises 2-10 layers of stirring paddles; Preferably, the circulation system includes a carbon dioxide circulation system and a slurry circulation system; the slurry circulation circulates small particle crystals inside the crystallizer through a circulation pump; the carbon dioxide circulation injects unreacted carbon dioxide back into the crystallizer for reaction through a compressor.

4. The preparation method according to claim 3, characterized in that: The stirring device is a three-layer stirring paddle, which includes a bottom stirring paddle, a middle stirring paddle and an upper stirring paddle from bottom to top; Preferably, the bottom stirring paddle is a six-blade disc stirring paddle, and the middle stirring paddle and the upper stirring paddle are both four-blade downward pressure propulsion stirring paddles; Preferably, the area below the bottom stirring paddle and between the bottom stirring paddle and the middle stirring paddle is the nucleation zone; Preferably, the area above the upper stirring paddle and between the upper stirring paddle and the middle stirring paddle is the growth zone.

5. The multi-stage continuous crystallization according to claim 1 or 4, characterized in that: The total carbon dioxide feed rate in step (1) is 2%-150% / min of the effective volume of the reaction crystallizer; Preferably, the carbon dioxide in step (1) is divided into 1-4 gas feeds; Preferably, the tail gas formed after the carbon dioxide reacts in step (1) is compressed and then re-enters the reaction crystallizer for recycling.

6. The multi-stage continuous crystallization according to claim 1, characterized in that: In step (2), the rate at which the slurry enters the primary cooling crystallizer is the same as the rate at which the slurry exits the primary cooling crystallizer; Preferably, the primary cooling crystallizer in step (2) comprises a main structure and a stirring device arranged in the main structure, and a guide tube is provided on the outside of the stirring device.

7. The multi-stage continuous crystallization according to claim 1, characterized in that: In step (3), the rate at which the slurry enters the secondary cooling crystallizer is the same as the rate at which the slurry exits the secondary cooling crystallizer; Preferably, the secondary cooling crystallizer in step (3) comprises a main structure and a stirring device arranged in the main structure, and a guide tube is provided on the outside of the stirring device.

8. The multi-stage continuous crystallization according to claim 1, characterized in that: The step (4) further comprises washing and drying the separated solids in sequence; Preferably, the cleaning solvent is a saturated baking soda solution, and the cleaning times are 1-3 times.

9. The multi-stage continuous crystallization method according to any one of claims 1 to 8 is used to prepare high-purity, large-size ellipsoidal sodium bicarbonate crystals; Preferably, the baking soda crystals have a particle size distribution of 300-450 μm, a purity of more than 99.5, and a repose angle of 25-35°.

10. Application of the high-purity large-size ellipsoidal sodium bicarbonate crystals according to claim 9 in the field of medicine.

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