Gluconolactone crystallization method

Abstract

The invention relates to the technical field of crystallization and purification, in particular to a glucolactone crystallization method. The method comprises the following steps: sequentially carrying out four-stage cooling crystallization on a raw material containing gluconic acid, wherein each stage of cooling crystallization comprises gradient cooling independently carried out in four temperature zones. According to the method provided by the invention, glucolactone crystals with regular and uniform particles, narrow particle size distribution range and high purity can be obtained, the yield is obviously higher than that of a conventional one-time crystallization method, especially when centrifugal separation is carried out, the separation effect is good, the water content of the separated material is low, the addition of crystal washing water in the separation process is less, and the mother liquor generation amount is less; and the production cost is greatly reduced.

Description

Technical Field

[0001] The invention relates to the technical field of crystallization and purification, and in particular to a method for crystallizing gluconolactone. Background Art

[0002] Gluconolactone, CAS registration number 90-80-2, is widely used in medicine, food, chemical industry and other fields. In the food field, gluconolactone is used as a protein coagulant, odor shielding agent, and acidity regulator; because gluconolactone has the function of chelating heavy metal ions, it can be used in chemical cleaning, leather, sewage treatment and other industries; in the material field, because gluconolactone is a glucose derivative that is easily biodegradable, it is used to produce biodegradable materials.

[0003] The preparation of gluconolactone generally involves first oxidizing glucose into gluconic acid or its salts, and then purifying, desalting, decolorizing, concentrating and crystallizing. The existing crystallization process of gluconolactone generally involves one-time crystallization or batch crystallization of the material after evaporation and concentration. In the one-time crystallization method, the crystallization temperature is difficult to control, pseudocrystals are easily generated during the crystallization process, or the crystals are small, which seriously affects the separation of the next step. The purity of the separated product is low. In order to solve the quality problem, the amount of washing water needs to be increased, resulting in a large amount of mother liquor, high cost of processing mother liquor, and ultimately high production cost; and the batch crystallization yield is low and the production efficiency is low. Summary of the invention

[0004] The purpose of the present invention is to overcome the problems of low yield and low purity of the gluconolactone crystallization method in the prior art and to provide a method for crystallizing gluconolactone.

[0005] In order to achieve the above object, the first aspect of the present invention provides a method for crystallizing gluconolactone, which comprises sequentially subjecting a raw material containing gluconic acid to four-stage cooling crystallization, wherein each stage of cooling crystallization comprises independently performing gradient cooling in four temperature zones, The temperatures of the four temperature zones of the first stage crystallization are: 75-71°C in the first temperature zone, 71-69°C in the second temperature zone, 69-67°C in the third temperature zone, and 67-65°C in the fourth temperature zone; The temperatures of the four temperature zones of the second stage crystallization are: the first temperature zone 65-61°C, the second temperature zone 61-59°C, the third temperature zone 59-57°C, and the fourth temperature zone 57-55°C; The temperatures of the four temperature zones of the third stage crystallization are: 55-53°C in the first temperature zone, 53-51°C in the second temperature zone, 51-48°C in the third temperature zone, and 48-45°C in the fourth temperature zone; The temperatures of the four temperature zones of the fourth stage crystallization are: 45-43°C in the first temperature zone, 43-41°C in the second temperature zone, 41-39°C in the third temperature zone, and 39-36°C in the fourth temperature zone.

[0006] By the above technical scheme, the method provided by the present invention can obtain gluconolactone crystals with regular, uniform, narrow and high particle size distribution range, and high purity, and the yield is significantly higher than the conventional primary crystallization method, especially when centrifugation is performed again, the separation effect is good, the moisture content of the material after separation is low, and the addition of crystal washing water in the separation process is small, and the amount of mother liquor produced is small, which greatly reduces the production cost. The method provided by the present invention has stable process and simple method, which is conducive to large-scale industrial production. In some preferred embodiments, the operation process of the method can realize comprehensive automation, automatic control of crystallization temperature classification and partitioning, automatic control of feed flow rate, and automatic control of stirring speed, which is conducive to reducing labor. DETAILED DESCRIPTION

[0007] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0008] The inventor of the present invention has found through research that the raw material containing gluconic acid is subjected to four-stage cooling steps in sequence, and each stage of cooling steps includes four temperature zones with decreasing temperature, and each adjacent temperature zone has a specific temperature difference, that is, a cooling crystallization method of four-stage sixteen temperature zones is adopted, which is more conducive to the formation of gradient crystallization, and each zone is in a slightly saturated state, which can effectively avoid the situation where the temperature drops sharply and causes the formation of fine crystals and pseudocrystals, and there is no situation where the temperature drops slowly and affects the crystallization cycle; the entire crystallization system creates a good environment for crystal growth, and in the crystallization process of each level and each zone, the crystal is formed in an oversaturated state, and as the temperature gradient is formed in each level and each zone, the crystal is slowly grown, and the crystallization effect and crystallization yield are better than the conventional primary crystallization method, and the generation of mother liquor and crystal washing water can be reduced, which greatly reduces the production cost. Thus, the present invention is obtained.

[0009] The first aspect of the present invention provides a method for crystallizing gluconolactone, which comprises subjecting a raw material containing gluconic acid to four cooling steps in sequence, each cooling step comprising four temperature zones with decreasing temperatures, and the temperature difference between each adjacent temperature zone is 2-6°C.

[0010] In order to improve the yield and make the crystal size more uniform, preferably, the temperature difference between each adjacent temperature zone is 2-4°C, more preferably 2-3°C.

[0011] Preferably, the temperature of the first stage crystallization is 75-65° C., the temperature of the second stage crystallization is 65-55° C., the temperature of the third stage is 55-45° C., and the temperature of the fourth stage crystallization is 45-36° C. The inventors of the present invention have found that when four-stage partitions are set and each stage maintains the above temperature for crystallization, it is more conducive to crystal formation and the crystal particle size is more uniform.

[0012] More preferably, the temperatures of the four temperature zones of the first stage crystallization are: 75-71°C in the first temperature zone, 71-69°C in the second temperature zone, 69-67°C in the third temperature zone, and 67-65°C in the fourth temperature zone.

[0013] More preferably, the temperatures of the four temperature zones of the second stage crystallization are: 65-61°C in the first temperature zone, 61-59°C in the second temperature zone, 59-57°C in the third temperature zone, and 57-55°C in the fourth temperature zone.

[0014] More preferably, the temperatures of the four temperature zones of the third stage crystallization are: 55-53°C in the first temperature zone, 53-51°C in the second temperature zone, 51-48°C in the third temperature zone, and 48-45°C in the fourth temperature zone.

[0015] More preferably, the temperatures of the four temperature zones of the fourth stage crystallization are: 45-43°C in the first temperature zone, 43-41°C in the second temperature zone, 41-39°C in the third temperature zone, and 39-36°C in the fourth temperature zone.

[0016] In the present invention, it is further preferred that the temperature of the first temperature zone of the first stage cooling is 72-71°C.

[0017] In the present invention, it is further preferred that the temperature of the second temperature zone of the first stage cooling is 70-69°C.

[0018] In the present invention, it is further preferred that the temperature of the third temperature zone of the first stage cooling is 68-67°C.

[0019] In the present invention, it is further preferred that the temperature of the fourth temperature zone of the first stage cooling is 66-65°C.

[0020] In the present invention, it is further preferred that the temperature of the first temperature zone of the second stage cooling is 62-61°C.

[0021] In the present invention, it is further preferred that the temperature of the second temperature zone of the second stage cooling is 60-59°C.

[0022] In the present invention, it is further preferred that the temperature of the third temperature zone of the second stage cooling is 58-57°C.

[0023] In the present invention, it is further preferred that the temperature of the fourth temperature zone of the second stage cooling is 56-55°C.

[0024] In the present invention, further preferably, the temperature of the first temperature zone of the third stage cooling is 55-53°C.

[0025] In the present invention, further preferably, the temperature of the second temperature zone of the third stage cooling is 51-52°C.

[0026] In the present invention, it is further preferred that the temperature of the third temperature zone of the third stage cooling is 49-48°C.

[0027] In the present invention, further preferably, the temperature of the fourth temperature zone of the third stage cooling is 46-45°C.

[0028] In the present invention, further preferably, the temperature of the first temperature zone of the fourth stage cooling is 44-43°C.

[0029] In the present invention, further preferably, the temperature of the second temperature zone of the fourth stage cooling is 42-41°C.

[0030] In the present invention, it is further preferred that the temperature of the third temperature zone of the fourth stage cooling is 40-39°C.

[0031] In the present invention, further preferably, the temperature of the fourth temperature zone of the fourth stage cooling is 37-36°C.

[0032] According to the present invention, a preferred embodiment is provided, wherein the method is carried out in a crystallization system including crystallization tanks connected in series, the tank bodies of the crystallization tanks at each level are not interconnected, the raw material is fed into the crystallization tank for the first level of crystallization, and passes through the crystallization tanks at each level in sequence; the crystallization tanks at each level are divided into four temperature zones from top to bottom; the raw material and the feed liquid in the crystallization process pass through each temperature zone of the crystallization tanks at each level from high to low. The feed liquid in each crystallization tank can automatically overflow the tank.

[0033] In the present invention, preferably, the method further comprises stirring with a stirrer during cooling at each stage, and when the blades of the stirrer are advection-type stirring, the crystallization yield can be further improved within the same crystallization time and the crystals can be made more uniform; in order to further improve the crystallization yield and make the crystal size distribution more uniform, a flat-blade stirrer is preferably used; further preferably, the flat-blade stirrer in each stage of cooling can be independently a two-blade, three-blade, four-blade or six-blade flat-blade stirrer; Preferably, the length of a single stirring blade of the flat-paddle agitator is 0.5-0.9R, wherein R is the radius of the crystallization tank for each stage of crystallization; the blades of the agitator used in the cooling steps at each stage may be different, for example, the length of a single stirring blade in the first stage cooling step may be 0.5R, the length of a single stirring blade in the second stage cooling step may be 0.7R, and the length of a single stirring blade in the third stage cooling step may be 0.9R. Those skilled in the art may make a selection according to actual needs.

[0034] In some more preferred embodiments, the flat-paddle agitators used in each stage of the cooling crystallization are all four-blade flat-paddle agitators; more preferably, the length of a single blade of the four-blade flat-paddle agitator is 0.9R.

[0035] In order to further improve the crystallization yield and make the crystal particle size distribution more uniform, preferably, the horizontal inclination angle of the stirring blade of the paddle agitator is 10-20 degrees, for example, it can be 10 degrees, 11 degrees, 12 degrees, 13 degrees, 14 degrees, 15 degrees, 16 degrees, 17 degrees, 18 degrees, 19 degrees, 20 degrees, and further preferably is 15 degrees and any range composed of any two of the above values ​​and any value in any range, and further preferably is 15 degrees.

[0036] More preferably, in each stage of crystallization, the stirring speed is 20-65 rpm. Further preferably, in each stage of crystallization, the stirring speed is independently 25-60 rpm.

[0037] In order to further improve the yield and make the crystal size more uniform, more preferably, in the first stage crystallization, the stirring speed is 35-60 rpm, for example, it can be 35 rpm, 45 rpm, 50 rpm, 55 rpm, 60 rpm, any range consisting of any two of the above values, and any value in any range.

[0038] In order to further improve the yield and make the crystal size more uniform, more preferably, in the second stage crystallization, the stirring speed is 35-45 rpm, for example, it can be 35 rpm, 37 rpm, 39 rpm, 42 rpm, 45 rpm, any range consisting of any two of the above values, and any value in any range.

[0039] In order to further improve the yield and make the crystal size more uniform, more preferably, in the third stage crystallization, the stirring speed is 30-40 rpm.

[0040] In order to further improve the yield and make the crystal size more uniform, more preferably, in the fourth stage crystallization, the stirring speed is 20-30 rpm, and more preferably 25-30 rpm.

[0041] In the present invention, the raw material is subjected to gradient cooling through sixteen temperature zones under a specific stirring speed and mode and a specific crystallization cycle, so that the yield of the obtained crystals is further improved and the particle size distribution is more uniform.

[0042] In the present invention, preferably, the crystallization system consists of four-stage, eight-stage or sixteen-stage crystallization tanks, more preferably consists of four-stage crystallization tanks.

[0043] In the present invention, the inventor divides the circulating water of a single crystallization tank into four independent circulation systems, namely four temperature zones.

[0044] Preferably, the temperature control method of each temperature zone in each stage of the crystallization tank includes independently controlling the temperature through circulating water.

[0045] The method provided by the present invention achieves cooling by maintaining the temperature of circulating water; by setting a cooling device in the crystallization tank, the temperature of circulating water in each temperature zone can be monitored in real time, and each temperature zone can be independently and automatically controlled. Furthermore, the circulating water reaches the required temperature by adding cold water. The inventors of the present invention have found that using this method to control the temperature of circulating water can further reduce the temperature fluctuation of the temperature zone compared with the prior art, and further improve the uniformity of crystal particle size distribution.

[0046] Those skilled in the art can also add discharge collection tanks, feed tanks and other devices as needed.

[0047] In the present invention, a crystallization tank having a specific number and temperature zones is used, so that the crystallization yield can be further improved and the obtained crystal particle size distribution is more uniform.

[0048] In the present invention, the discharging method of each crystallizer is automatic material connection through the liquid level difference, and the material connection method is low out and high in. In addition, the material connection pipelines between the crystallizers can be insulated according to conventional means in the art.

[0049] In the present invention, preferably, the feed flow rate of the raw material is 1-2 m³ / h.

[0050] Preferably, the purity of the raw material is above 95%, more preferably above 98%.

[0051] In the present invention, the concentration of the raw material can be selected within a larger range, preferably 800-960 g / kg, for example, 800 g / kg, 820 g / kg, 840 g / kg, 860 g / kg, 880 g / kg, 900 g / kg, 920 g / kg, 940 g / kg, 960 g / kg, any range consisting of any two of the above values, and any value in any range.

[0052] More preferably, the second stage crystallization is started when the feed liquid in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the second stage crystallization.

[0053] More preferably, the third stage crystallization is started when the feed liquid in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the third stage crystallization.

[0054] More preferably, the third stage crystallization is started when the feed liquid in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the third stage crystallization.

[0055] In the present invention, preferably, the method further comprises adding gluconolactone seed crystals after the first stage crystallization temperature is stabilized when the raw materials are first added.

[0056] More preferably, 2-4 g of gluconolactone seed crystals are added per kilogram of gluconic acid in the raw material.

[0057] According to the method provided by the present invention, raw materials can be continuously added for cooling crystallization, the gluconolactone seed crystals do not need to be repeatedly added to achieve high-yield crystallization, the amount of seed crystals used is small, and the particle size uniformity of the obtained crystals is good.

[0058] In the present invention, preferably, the cycle of the cooling crystallization is 20-28h, wherein the time of each level of cooling crystallization can be independently 2-12h, for example, 2h, 4h, 6h, 8h, 10h, 12h and any range consisting of any two of the above values ​​and any value in any range.

[0059] According to the method provided by the present invention, the method further comprises separation after crystallization, and optionally washing the crystals. The separation condition may be 2500-3000 rpm / min, and the centrifuge may be a centrifuge conventionally used in the art, preferably a horizontal screw centrifuge, and those skilled in the art may also select one according to actual needs.

[0060] Optionally, the crystal washing is a conventional method in the art, and those skilled in the art can select it according to actual needs.

[0061] The present invention will be described in detail below by way of examples. However, the embodiments of the present invention are not limited thereto. In the following examples, unless otherwise specified, the materials and equipment used can be obtained commercially.

[0062] Example 1 This example is used to illustrate the method of crystallization.

[0063] A four-stage series crystallization tank is used. Each crystallization tank is equipped with a four-blade flat-paddle agitator with a horizontal inclination angle of 15 degrees. The length of a single blade of the four-blade flat-paddle agitator is 0.9R, where R is the radius of the crystallization tank. The temperature of the crystallization tank is automatically controlled by controlling the circulating water temperature, and the circulating water of a single crystallization tank is divided into four independent circulation systems, i.e., four temperature zones. Each temperature zone is automatically temperature controlled by adding cold water. The discharging method of each crystallization tank is: automatic material connection through liquid level difference, the material connection method is low out and high in, and the material connection pipeline is insulated.

[0064] The specific crystallization steps are as follows: First stage crystallization: add the concentrated gluconic acid to the first stage crystallization tank at a rate of 1.5m³ / h, the gluconic acid concentration is 900g / kg, the purity is 99%, after the first stage crystallization tank is filled with material, start stirring, set the stirring speed to 50 rpm, set the circulating water temperature of the first zone to 71°C, start the circulating pump, set the temperature of the second zone to 69°C, open the circulating water automatic control valve, set the temperature of the third zone to 67°C, open the circulating water automatic control valve, set the temperature of the fourth zone to 65°C, open the circulating water automatic control valve, when the temperature of each zone is stable, add gluconolactone as a seed, add 5g of gluconolactone for every 1kg of gluconic acid, and run continuously for 12h; Second stage crystallization: after running for 12 hours, the materials in the first stage crystallization tank automatically flow into the second stage crystallization tank. When the liquid level of the second stage crystallization tank reaches the overflow port, the first stage crystallization tank stops feeding, the agitator of the second stage crystallization tank is turned on, the agitator speed is set to 40 rpm, the temperature of each zone of the second stage is set to 61°C in the first zone, 59°C in the second zone, 57°C in the third zone, and 55°C in the fourth zone, and the temperature is set to run stably for 2 hours; then the materials flow into the next stage crystallization tank; Third stage crystallization: when the feed volume of the third stage crystallizer accounts for about 85% of the volume of the third stage crystallizer, start the third stage stirring, set the stirring speed to 30 rpm, set the temperature of each zone to 53°C for the cooling water in the first zone, 51°C for the second zone, 48°C for the third zone, and 45°C for the fourth zone, and open the circulating water automatic control valve; Fourth-stage crystallization: When the third-stage crystallization tank is full, the material automatically flows into the fourth-stage crystallization tank. When the feed volume of the fourth-stage crystallization tank accounts for about 85% of the volume of the fourth-stage crystallization tank, the fourth-stage stirring is turned on and the fourth-stage stirring speed is set to 28 rpm. When the material tank is full, the first-stage feeding is stopped and the temperature of each zone of the fourth stage is set to 43°C for the first zone, 41°C for the second zone, 39°C for the third zone, and 36°C for the fourth zone. The entire fourth-stage crystallization tank runs continuously for 24 hours. Continuous discharging: When the four-stage crystallization tank is filled with material and runs continuously for 24 hours, the first-stage feeding is opened, the feeding flow rate is 1.5m³ / h, and the discharging overflows automatically. The discharging collection tank adopts a constant temperature tank. When the constant temperature tank reaches a certain volume, it is pumped into the horizontal screw centrifuge for solid-liquid separation. A screen is set at the centrifuge. The separation condition is: 2700rpm / min.

[0065] Example 2 The method of Example 1 is followed, except that the temperatures of each stage of crystallization are different, as shown in Table 1.

[0066] Example 3 The method of Example 1 is followed, except that a three-blade propeller stirrer is used for stirring.

[0067] Example 4 The method of Example 1 is different in that the rotation speeds of each stage of crystallization are different, as follows: The first stage stirring speed is 40 rpm. The second stage stirring speed is 30 rpm. The third stage stirring speed is 25 rpm. The fourth stage stirring speed is 20 rpm.

[0068] Comparative Example 1 The method of Example 1 is followed, except that the temperatures of each stage of crystallization are different, as shown in Table 1.

[0069] Comparative Example 2 The method of Example 1 is followed, except that only one temperature zone is set for each stage of crystallization. The temperature of each temperature zone is shown in Table 1.

[0070] Comparative Example 3 The method of Example 1 was followed, except that only two temperature zones were set for each stage of crystallization. The temperature of each temperature zone is shown in Table 1.

[0071] Comparative Example 4 The method of Example 1 was followed, except that only three temperature zones were set for each stage of crystallization. The temperature of each temperature zone is shown in Table 1.

[0072] Comparative Example 5 According to the method of Example 1, the difference is that After the raw materials and gluconolactone seed crystals were put into the crystallization tank, the crystallization was carried out by stirring at 50° C. for 24 hours with a stirring speed of 50 rpm.

[0073] Table 1

[0074] Test Example 1 1. Detect the particle size distribution of the separated crystals. Use 24-mesh and 32-mesh sieves to determine the proportion of the weight of crystals with a particle size in the range of 24-32 mesh to the total mass of the crystals. The higher the proportion, the better the uniformity.

[0075] 2. Detect the yield of the crystals in the collection tank.

[0076] Yield = mass of crystals in the collection tank / (mass of gluconolactone seed crystals + mass of gluconolactone obtained by converting the molar amount of gluconic acid in the raw material) 3. The purity detection method is as follows: prepare 10.0 mg / mL gluconolactone standard solution (standard product purchased from Sigma, USA), and dilute it into solutions with concentrations of 10.0 mg / mL, 8.0 mg / mL, 6.0 mg / mL, 4.0 mg / mL, 2.0 mg / mL, 1.0 mg / mL, 0.5 mg / mL and 0.25 mg / mL. Use a high performance liquid chromatography system to measure the peak area of ​​the gluconolactone standard solution, and draw a standard curve with the peak area (average value of 3 sets of parallel samples) as the ordinate and the solution concentration as the abscissa, and analyze the content of gluconolactone in the format example and the comparative example according to the standard curve.

[0077] Table 2

[0078] It can be seen from Table 1 that, using the method provided by the present invention, the particle size distribution of the obtained crystals is more uniform, the yield is higher, and under the same separation conditions, the mother liquor adhering to the surface of the crystals is less and the purity is higher. Under the same separation conditions, solid-liquid separation can be achieved in a shorter time to obtain the product.

[0079] Comparative Example 5 uses conventional methods for cooling crystallization, and the resulting crystals have fine crystals and pseudocrystals, and Comparative Examples 2-3 also have more fine crystals and pseudocrystals. Comparative Examples 1 and 4 use multiple temperature zones, and at the same crystallization time as Example 1, the resulting crystals have less quality and lower purity.

[0080] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for crystallizing gluconolactone, characterized in that: The method comprises sequentially subjecting a raw material containing gluconic acid to four-stage cooling crystallization, wherein each stage of cooling crystallization comprises independently performing gradient cooling in four temperature zones. The temperatures of the four temperature zones of the first stage crystallization are: 75-71°C in the first temperature zone, 71-69°C in the second temperature zone, 69-67°C in the third temperature zone, and 67-65°C in the fourth temperature zone; The temperatures of the four temperature zones of the second stage crystallization are: the first temperature zone 65-61°C, the second temperature zone 61-59°C, the third temperature zone 59-57°C, and the fourth temperature zone 57-55°C; The temperatures of the four temperature zones of the third stage crystallization are: 55-53°C in the first temperature zone, 53-51°C in the second temperature zone, 51-48°C in the third temperature zone, and 48-45°C in the fourth temperature zone; The temperatures of the four temperature zones of the fourth stage crystallization are: 45-43°C in the first temperature zone, 43-41°C in the second temperature zone, 41-39°C in the third temperature zone, and 39-36°C in the fourth temperature zone.

2. The method according to claim 1, wherein: The method further comprises stirring with a flat-paddle stirrer during cooling at each stage; Preferably, the paddle stirrer in each cooling step is independently a two-blade, three-blade, four-blade or six-blade paddle stirrer; Preferably, the length of a single stirring blade of the flat-paddle stirrer is 0.5R -0.9R; Preferably, the horizontal inclination angle of the stirring blade of the flat-paddle stirrer is 10-20 degrees, more preferably 15 degrees; More preferably, the flat-paddle agitator is a four-blade flat-paddle agitator; More preferably, the length of a single blade of the four-blade flat-paddle agitator is 0.9R.

3. The method according to claim 2, wherein: In each stage of crystallization, the stirring speed is independently 20-65 rpm.

4. The method according to claim 3, wherein: In each stage of crystallization, the stirring speed is independently 25-60 rpm; Preferably, in the first stage crystallization, the stirring speed is 35-60 rpm; Preferably, in the second stage crystallization, the stirring speed is 35-45 rpm; Preferably, in the third stage crystallization, the stirring speed is 30-40 rpm; Preferably, in the fourth stage crystallization, the stirring speed is 20-30 rpm, more preferably 25-30 rpm.

5. The method according to any one of claims 1 to 4, wherein: The method is carried out in a crystallization system comprising at least four stages of crystallization tanks connected in series, the tank bodies of the crystallization tanks at each stage are not interconnected, and the raw material is fed into the crystallization tank for the first stage of crystallization and passes through the crystallization tanks at each stage in sequence.

6. The method according to claim 5, wherein: The temperature control method of each temperature zone in each level of the crystallization tank includes independently controlling the temperature through circulating water; Preferably, the circulating water is brought to the desired temperature by adding cold water.

7. The method according to any one of claims 1 to 6, wherein: The method further comprises adding gluconolactone seed crystals when the temperature of the raw material is 65-75°C; Preferably, 2-4 g of gluconolactone seed crystals are added for each kilogram of gluconic acid in the raw material.

8. The method according to any one of claims 1 to 7, wherein: The purity of the raw material is above 95%, more preferably above 98%; Preferably, the concentration of the raw material is 800-960 g / kg.

9. The method according to any one of claims 5 to 8, wherein: When the feed liquid in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the second stage crystallization, the second stage crystallization is started; Preferably, the third stage crystallization is started when the feed liquid in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the third stage crystallization; Preferably, when the liquid material in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the fourth stage crystallization, the fourth stage crystallization is started.

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