A method for crystallizing glucono delta-lactone

Abstract

The present invention relates to the technical field of crystallization purification, and particularly relates to a method for crystallizing glucono delta-lactone. The method includes subjecting a raw material containing gluconic acid to four-stage cooling crystallization in sequence, and each stage of cooling crystallization includes independently performing gradient cooling in four temperature zones. The method provided by the present invention can obtain glucono delta-lactone crystals with regular, uniform particles, a narrow particle size distribution range and high purity. The yield is significantly higher than that of the conventional single crystallization method. Especially during centrifugal separation, the separation effect is good, the moisture content of the separated material is low, and the amount of washing crystal water added during the separation process is small, and the amount of mother liquor generated is small, greatly reducing the production cost.

Description

Technical Field

[0001] The present invention relates to the technical field of crystallization purification, and particularly relates to a method for crystallizing glucono delta-lactone. Background Art

[0002] Glucono delta-lactone, with the CAS registration number 90-80-2, is widely used in various fields such as medicine, food, and chemical industry. In the food field, glucono delta-lactone is used as a protein coagulant, off-flavor masking agent, and sour taste regulator; due to its function of chelating heavy metal ions, it can be used in industries such as chemical cleaning, leather, and sewage treatment; in the material field, since glucono delta-lactone is a glucose derivative and is easily biodegradable, it is used to produce biodegradable materials.

[0003] The preparation of glucono delta-lactone generally first oxidizes glucose into gluconic acid or its salts, and then undergoes purification, desalting, decolorization, concentration, and crystallization. The existing crystallization process of glucono delta-lactone is generally one-time crystallization or batch crystallization of the evaporated and concentrated material. In the one-time crystallization method, the crystallization temperature is difficult to control, and pseudo-crystals are likely to appear during the crystallization process, or the crystals are fine, seriously affecting the separation in the next process. The purity of the separated product is low. To solve the quality problem, it is necessary to increase the amount of washing water, resulting in a large amount of mother liquor production and high cost for treating the mother liquor, ultimately leading to high production costs; 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 in the existing glucono delta-lactone crystallization method, and provide a method for crystallizing glucono delta-lactone.

[0005] To achieve the above purpose, the first aspect of the present invention provides a method for crystallizing glucono delta-lactone, which includes subjecting the raw material containing gluconic acid to four-stage cooling crystallization in sequence, and each stage of cooling crystallization includes independently performing gradient cooling in four temperature zones.

[0006] Among them, the temperatures of the four temperature zones of the first-stage crystallization are respectively: the first temperature zone is 75 - 71 °C, the second temperature zone is 71 - 69 °C, the third temperature zone is 69 - 67 °C, and the fourth temperature zone is 67 - 65 °C;

[0007] The temperatures of the four temperature zones of the second-stage crystallization are respectively: the first temperature zone is 65 - 61 °C, the second temperature zone is 61 - 59 °C, the third temperature zone is 59 - 57 °C, and the fourth temperature zone is 57 - 55 °C;

[0008] The temperatures of the four temperature zones of the third-stage crystallization are respectively: the first temperature zone is 55 - 53 °C, the second temperature zone is 53 - 51 °C, the third temperature zone is 51 - 48 °C, and the fourth temperature zone is 48 - 45 °C;

[0009] The temperatures of the four temperature zones for the fourth-stage crystallization are as follows: the first temperature zone is 45 - 43 °C, the second temperature zone is 43 - 41 °C, the third temperature zone is 41 - 39 °C, and the fourth temperature zone is 39 - 36 °C.

[0010] Through the above technical solution, the method provided by the present invention can obtain glucono delta-lactone crystals with regular, uniform particles, a narrow particle size distribution range, and high purity. The yield is significantly higher than that of the conventional single-stage crystallization method. Especially during centrifugal separation, the separation effect is good, the moisture content of the separated material is low, and less washing water is added during the separation process, resulting in less mother liquor production, greatly reducing the production cost. The method provided by the present invention has a stable process and is simple, which is beneficial to large-scale industrial production. In some preferred embodiments, the operation process of the method can achieve full automation, with automatic control of the crystallization temperature in different zones, automatic control of the feeding flow rate, and automatic control of the stirring speed, which is beneficial to reducing the labor intensity. Detailed Embodiments

[0011] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0012] The inventors of the present invention found through research that a raw material containing gluconic acid is successively subjected to four-stage cooling steps. Each stage of the cooling step includes four temperature zones with decreasing temperatures, and there is a specific temperature difference between adjacent temperature zones, that is, a four-stage and sixteen-temperature-zone cooling crystallization method is adopted, which is more conducive to forming gradient crystallization. Moreover, each zone is in a slightly supersaturated state, which can effectively avoid the formation of fine crystals and pseudo-crystals caused by a sharp drop in temperature, and there is no situation where the slow drop in temperature affects the crystallization cycle; the entire crystallization system creates a good environment for crystal growth. During the crystallization process in each stage and each zone, the crystals are formed in a supersaturated state. With the formation of a temperature gradient in each stage and each zone, the crystals grow slowly, and the crystallization effect and crystallization yield are better than those of the conventional single-stage crystallization method, and it can reduce the production of mother liquor and washing water, greatly reducing the production cost. Thus, the present invention is obtained.

[0013] The first aspect of the present invention provides a method for glucono delta-lactone crystallization, which includes successively subjecting a raw material containing gluconic acid to four-stage cooling steps. Each stage of the cooling step includes four temperature zones with decreasing temperatures, and the temperature difference between adjacent temperature zones is 2 - 6 °C.

[0014] To improve the yield and make the crystal particle size more uniform, preferably, the temperature difference between adjacent temperature zones is 2 - 4 °C, and more preferably 2 - 3 °C.

[0015] 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 zoning is set and crystallization is carried out while maintaining the above temperatures at each stage, it is more conducive to crystal formation and the crystal particle size is more uniform.

[0016] More preferably, the temperatures of the four temperature zones of the first-stage crystallization are respectively: the first temperature zone is 75 - 71°C, the second temperature zone is 71 - 69°C, the third temperature zone is 69 - 67°C, and the fourth temperature zone is 67 - 65°C.

[0017] More preferably, the temperatures of the four temperature zones of the second-stage crystallization are respectively: the first temperature zone is 65 - 61°C, the second temperature zone is 61 - 59°C, the third temperature zone is 59 - 57°C, and the fourth temperature zone is 57 - 55°C.

[0018] More preferably, the temperatures of the four temperature zones of the third-stage crystallization are respectively: the first temperature zone is 55 - 53°C, the second temperature zone is 53 - 51°C, the third temperature zone is 51 - 48°C, and the fourth temperature zone is 48 - 45°C.

[0019] More preferably, the temperatures of the four temperature zones of the fourth-stage crystallization are respectively: the first temperature zone is 45 - 43°C, the second temperature zone is 43 - 41°C, the third temperature zone is 41 - 39°C, and the fourth temperature zone is 39 - 36°C.

[0020] In the present invention, further preferably, the temperature of the first temperature zone of the first-stage cooling is 72 - 71°C.

[0021] In the present invention, further preferably, the temperature of the second temperature zone of the first-stage cooling is 70 - 69°C. ]>

[0022] In the present invention, further preferably, the temperature of the third temperature zone of the first-stage cooling is 68 - 67°C.

[0023] In the present invention, further preferably, the temperature of the fourth temperature zone of the first-stage cooling is 66 - 65°C.

[0024] In the present invention, further preferably, the temperature of the first temperature zone of the second-stage cooling is 62 - 61°C.

[0025] In the present invention, further preferably, the temperature of the second temperature zone of the second-stage cooling is 60 - 59°C.

[0026] In the present invention, further preferably, the temperature of the third temperature zone of the second-stage cooling is 58 - 57°C.

[0027] In the present invention, further preferably, the temperature of the fourth temperature zone of the second-stage cooling is 56 - 55°C.

[0028] In the present invention, further preferably, the temperature of the first temperature zone of the third-stage cooling is 55 - 53 °C.

[0029] In the present invention, further preferably, the temperature of the second temperature zone of the third-stage cooling is 51 - 52 °C.

[0030] In the present invention, further preferably, the temperature of the third temperature zone of the third-stage cooling is 49 - 48 °C.

[0031] In the present invention, further preferably, the temperature of the fourth temperature zone of the third-stage cooling is 46 - 45 °C.

[0032] In the present invention, further preferably, the temperature of the first temperature zone of the fourth-stage cooling is 44 - 43 °C.

[0033] In the present invention, further preferably, the temperature of the second temperature zone of the fourth-stage cooling is 42 - 41 °C.

[0034] In the present invention, further preferably, the temperature of the third temperature zone of the fourth-stage cooling is 40 - 39 °C.

[0035] In the present invention, further preferably, the temperature of the fourth temperature zone of the fourth-stage cooling is 37 - 36 °C.

[0036] According to a preferred embodiment provided by the present invention, the method is carried out in a crystallization system including crystallization tanks connected in series in sequence. The tank bodies of each stage of crystallization tanks are not connected to each other. The raw material is fed into the crystallization tank for the first-stage crystallization and passes through each stage of crystallization tanks in sequence. Each stage of crystallization tank is divided into four temperature zones from top to bottom. The raw material and the liquid in the crystallization process pass through each temperature zone of each stage of crystallization tank from high to low. The liquid in each crystallization tank can overflow from the tank automatically.

[0037] In the present invention, preferably, the method further includes using a stirrer for stirring during each stage of cooling. When the paddle of the stirrer is a laminar flow type stirrer, the crystallization yield can be further improved and the crystals can be made more uniform within the same crystallization time. In order to further improve the crystallization yield and make the crystal size distribution more uniform, a flat paddle stirrer is preferably used. Further preferably, in each stage of cooling step, the flat paddle stirrer can be independently a two-blade, three-blade, four-blade or six-blade flat paddle stirrer;

[0038] Preferably, the single - blade length of the stirring blade of the flat - blade stirrer is 0.5 - 0.9R, where R is the radius of the crystallization tank for each - stage crystallization; the blades of the stirrers used in each cooling step can be different. For example, the single - blade length of the stirring blade in the first - stage cooling step can be 0.5R, the single - blade length of the stirring blade in the second - stage cooling step can be 0.7R, and the single - blade length of the stirring blade in the third - stage cooling step can be 0.9R. Those skilled in the art can select according to actual needs.

[0039] In some more preferred embodiments, the flat - blade stirrers used in each - stage cooling crystallization are all four - blade flat - blade stirrers; more preferably, the single - blade length of the blades of the four - blade flat - blade stirrer is 0.9R.

[0040] To further improve the crystallization yield and make the crystal size distribution more uniform, preferably, the horizontal inclination angle of the stirring blade of the paddle stirrer 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. Further preferably, it is 15 degrees and any range composed of any two of the above values and any value in any range. Further preferably, it is 15 degrees.

[0041] More preferably, in each - stage crystallization, the stirring speed is 20 - 65 revolutions per minute. Further preferably, in each - stage crystallization, the stirring speeds are independently 25 - 60 revolutions per minute.

[0042] 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 revolutions per minute, for example, it can be 35 revolutions per minute, 45 revolutions per minute, 50 revolutions per minute, 55 revolutions per minute, 60 revolutions per minute and any range composed of any two of the above values and any value in any range.

[0043] 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 revolutions per minute, for example, it can be 35 revolutions per minute, 37 revolutions per minute, 39 revolutions per minute, 42 revolutions per minute, 45 revolutions per minute and any range composed of any two of the above values and any value in any range.

[0044] 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 revolutions per minute.

[0045] 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 revolutions per minute, further preferably 25 - 30 revolutions per minute.

[0046] In the present invention, the raw materials are subjected to gradient cooling through sixteen temperature zones under the cooperation of a specific stirring speed and mode and a specific crystallization period, so that the crystal yield is further improved and the particle size distribution is more uniform.

[0047] In the present invention, preferably, the crystallization system is composed of four-stage, eight-stage or sixteen-stage crystallization tanks, and more preferably composed of four-stage crystallization tanks.

[0048] In the present invention, the inventor divides the circulating water of a single crystallization tank into four independent circulating systems, that is, four temperature zones.

[0049] Preferably, the temperature control mode of each temperature zone in each stage of crystallization tank includes independently controlling the temperature through circulating water.

[0050] The method provided by the present invention realizes cooling by maintaining the temperature of the circulating water; by setting a cooling device in the crystallization tank, the temperature of the circulating water in each temperature zone can be monitored in real time, and independent automatic temperature control of each temperature zone can be realized. Further, the circulating water reaches the required temperature by adding cold water. The inventor of the present invention has found that using this method to control the temperature of the circulating water can further reduce the temperature fluctuation of the temperature zone and further improve the uniformity of the crystal particle size distribution compared with the prior art.

[0051] Those skilled in the art can also add devices such as a discharge collection tank and a feed tank according to needs.

[0052] In the present invention, by using a crystallization tank with temperature zones having a specific number and temperature, the crystallization yield can be further improved and the particle size distribution of the obtained crystals is more uniform.

[0053] In the present invention, the discharge mode of each stage of crystallization tank is automatic serial feeding through the liquid level difference, and the serial feeding mode is low discharge and high inlet. And, the serial feeding pipeline between the crystallization tanks can be insulated by conventional means in the art.

[0054] In the present invention, preferably, the feeding flow rate of the raw materials is 1-2 m³ / h.

[0055] Preferably, the purity of the raw materials is above 95%, and more preferably above 98%.

[0056] In the present invention, the concentration of the raw materials can be selected within a relatively large range, preferably 800-960 g / kg, for example, it can be 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, and any range composed of any two of the above values and any value within any range.

[0057] More preferably, when the 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.

[0058] More preferably, when the liquid in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the third-stage crystallization, the third-stage crystallization is started.

[0059] More preferably, when the liquid in the crystallization process accounts for 85±5% of the volume of the crystallization tank for the third-stage crystallization, the third-stage crystallization is started.

[0060] In the present invention, preferably, the method further includes adding glucono delta-lactone seeds after the first feeding of raw materials when the temperature of the first-stage crystallization is stable.

[0061] More preferably, 2-4 g of glucono delta-lactone seeds are added relative to each kilogram of gluconic acid in the raw materials.

[0062] According to the method provided by the present invention, raw materials can be continuously added for cooling crystallization. The glucono delta-lactone seeds can achieve high-yield crystallization without repeated addition, with a small amount of seeds used and good particle size uniformity of the obtained crystals.

[0063] In the present invention, preferably, the period of the cooling crystallization is 20-28 h, wherein the time of each stage of the cooling crystallization can be independently 2-12 h, such as 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, and any range composed of any two of the above values and any value in any range.

[0064] According to the method provided by the present invention, wherein the method further includes separation after crystallization and optional crystal washing. The conditions for the separation can be 2500-3000 rpm / min. The centrifuge can be a centrifuge commonly used in the art, preferably a horizontal spiral centrifuge, and those skilled in the art can also make a choice according to actual needs.

[0065] Optionally, the crystal washing is a conventional method in the art, and those skilled in the art can make a choice according to actual needs.

[0066] The present invention will be described in detail below through 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 through commercial purchase.

[0067] Example 1

[0068] This example is used to illustrate the crystallization method.

[0069] It is carried out using a four-stage series crystallization tank. Each stage of the crystallization tank is equipped with a four-blade flat paddle agitator with a horizontal inclination angle of 15 degrees. The single-blade length 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 fully automatically controlled by controlling the temperature of the circulating water. The circulating water of a single crystallization tank is divided into four independent circulation systems, that is, four temperature zones. Each temperature zone realizes automatic temperature control by adding cold water. The discharging method of each stage of the crystallization tank is as follows: automatic serial feeding through the liquid level difference. The serial feeding method is low-out and high-in, and the serial feeding pipeline is insulated.

[0070] The specific crystallization steps are as follows:

[0071] First-stage crystallization: The concentrated gluconic acid is fed into the first-stage crystallization tank at a speed of 1.5 m³ / h. The concentration of gluconic acid is 900 g / kg and the purity is 99%. After the first-stage crystallization tank is filled with materials, start the agitation, set the agitation speed to 50 revolutions per minute, set the temperature of the circulating water in the first zone to 71 °C, start the circulation pump, set the temperature of the second zone to 69 °C, open the automatic control valve of the circulating water, set the temperature of the third zone to 67 °C, open the automatic control valve of the circulating water, set the temperature of the fourth zone to 65 °C, open the automatic control valve of the circulating water. When the temperatures of each zone are stable, add glucono delta-lactone as the crystal seed. Add 5 g of glucono delta-lactone per 1 kg of gluconic acid and run continuously for 12 h;

[0072] Second-stage crystallization: After running for 12 h, 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. Start the agitator of the second-stage crystallization tank, set the agitation speed to 40 revolutions per minute, set the temperatures of each zone in the second stage, the temperature of the first zone is 61 °C, the temperature of the second zone is set to 59 °C, the temperature of the third zone is set to 57 °C, the temperature of the fourth zone is set to 55 °C, and run stably for 2 h; then the materials flow into the next-stage crystallization tank;

[0073] Third-stage crystallization: When the feeding volume of the third-stage crystallization tank accounts for about 85% of the volume of the third-stage crystallization tank, start the agitation of the third stage, set the agitation speed to 30 revolutions per minute, set the temperatures of each zone as the cooling water in the first zone is at a temperature of 53 °C, the temperature of the second zone is set to 51 °C, the temperature of the third zone is set to 48 °C, the temperature of the fourth zone is set to 45 °C, and open the automatic control valve of the circulating water;

[0074] Fourth-stage crystallization: When the third-stage crystallization tank is full, the materials automatically flow into the fourth-stage crystallization tank. When the feeding volume of the fourth-stage crystallization tank accounts for about 85% of the volume of the fourth-stage crystallization tank, start the agitation of the fourth stage, set the agitation speed of the fourth stage to 28 revolutions per minute. When the materials are full in the tank, stop the feeding of the first stage, and set the temperatures of each zone in the fourth stage as the temperature of the first zone is 43 °C, the temperature of the second zone is set to 41 °C, the temperature of the third zone is set to 39 °C, the temperature of the fourth zone is set to 36 °C, and the entire four-stage crystallization tank runs continuously for 24 h;

[0075] Continuous discharging: After the fourth-stage crystallization tank is filled with materials and operates continuously for 24 hours, the first-stage feeding is started, with a feeding flow rate of 1.5 m³ / h. The discharging is automatically overflowed, and the discharging collection tank is a constant-temperature tank. When the constant-temperature tank reaches a certain volume, it is pumped into a horizontal scroll centrifuge for solid-liquid separation. A screen is set at the centrifuge, and the separation condition is 2700 rpm / min.

[0076] Example 2

[0077] It is carried out according to the method of Example 1, except that the temperatures of each stage of crystallization are different, as shown in Table 1 specifically.

[0078] Example 3

[0079] It is carried out according to the method of Example 1, except that a three-blade propeller agitator is used for stirring.

[0080] Example 4

[0081] According to the method of Example 1, the difference is that the rotation speeds of each stage of crystallization are different, specifically as follows:

[0082] The stirring rotation speed of the first stage is 40 revolutions per minute,

[0083] The stirring rotation speed of the second stage is 30 revolutions per minute,

[0084] The stirring rotation speed of the third stage is 25 revolutions per minute,

[0085] The stirring rotation speed of the fourth stage is 20 revolutions per minute.

[0086] Comparative Example 1

[0087] It is carried out according to the method of Example 1, except that the temperatures of each stage of crystallization are different, as shown in Table 1 specifically.

[0088] Comparative Example 2

[0089] It is carried out according to the method of Example 1, except that only one temperature zone is set for each stage of crystallization, and the temperatures of each temperature zone are shown in Table 1.

[0090] Comparative Example 3

[0091] It is carried out according to the method of Example 1, except that only 2 temperature zones are set for each stage of crystallization, and the temperatures of each temperature zone are shown in Table 1.

[0092] Comparative Example 4

[0093] It is carried out according to the method of Example 1, except that only 3 temperature zones are set for each stage of crystallization, and the temperatures of each temperature zone are shown in Table 1.

[0094] Comparative Example 5

[0095] According to the method of Example 1, the difference is that

[0096] After the raw materials and glucono delta-lactone seeds are put into the crystallization tank, crystallization is carried out with stirring at 50 °C for 24 h, and the stirring speed is 50 revolutions per minute.

[0097] Table 1

[0098]

[0099] Test Example 1

[0100] 1. To detect the particle size distribution of the separated crystals, 24-mesh and 32-mesh sieves are used to determine the proportion of the weight of the crystals with a particle size in the range of 24 - 32 meshes in the total mass of the crystals. The higher this proportion, the better the uniformity.

[0101] 2. Detect the yield of the crystals in the collection tank.

[0102] Yield = mass of crystals in the collection tank / (mass of glucono delta-lactone seeds + mass of glucono delta-lactone obtained by converting the molar amount of gluconic acid in the raw materials)

[0103] 3. The detection method for purity is as follows: Prepare a 10.0 mg / mL glucono delta-lactone standard solution (the standard product is purchased from Sigma, USA), and successively 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 glucono delta-lactone standard solution. Take the peak area (average value of 3 parallel samples) as the ordinate and the solution concentration as the abscissa to plot a standard curve, and analyze the content of glucono delta-lactone in the format examples and comparative examples according to the standard curve.

[0104] Table 2

[0105]

[0106] As can be seen from Table 1, using the method provided by the present invention, the obtained crystals have a more uniform particle size distribution, a higher yield, less mother liquor adhering to the crystal surface under the same separation conditions, a higher purity, and can achieve solid-liquid separation to obtain the product in a shorter time under the same separation conditions.

[0107] In Comparative Example 5, cooling crystallization was carried out by a conventional method, and fine crystals and pseudo-crystals appeared in the obtained crystals. More fine crystals and pseudo-crystals also appeared in Comparative Examples 2 - 3. In Comparative Examples 1 and 4, multiple temperature zones were used, and under the same crystallization time as in Example 1, the obtained crystals had a small mass and a lower purity.

[0108] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should equally be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for crystallizing glucono delta-lactone, characterized in that, The method includes subjecting the raw material containing gluconic acid to four-stage cooling crystallization in sequence. Each stage of cooling crystallization includes independently performing gradient cooling in four temperature zones. Among them, the temperatures of the four temperature zones in the first-stage crystallization are respectively: the first temperature zone is 75 - 71 °C, the second temperature zone is 71 - 69 °C, the third temperature zone is 69 - 67 °C, and the fourth temperature zone is 67 - 65 °C; The temperatures of the four temperature zones in the second-stage crystallization are respectively: the first temperature zone is 65 - 61 °C, the second temperature zone is 61 - 59 °C, the third temperature zone is 59 - 57 °C, and the fourth temperature zone is 57 - 55 °C; The temperatures of the four temperature zones in the third-stage crystallization are respectively: the first temperature zone is 55 - 53 °C, the second temperature zone is 53 - 51 °C, the third temperature zone is 51 - 48 °C, and the fourth temperature zone is 48 - 45 °C; The temperatures of the four temperature zones in the fourth-stage crystallization are respectively: the first temperature zone is 45 - 43 °C, the second temperature zone is 43 - 41 °C, the third temperature zone is 41 - 39 °C, and the fourth temperature zone is 39 - 36 °C.

2. The method according to claim 1, wherein, The method further includes using a flat paddle stirrer for stirring during each stage of cooling; In each stage of cooling step, the flat paddle stirrer is independently a two-blade, three-blade, four-blade or six-blade flat paddle stirrer; The single-blade length of the stirring paddle of the flat paddle stirrer is 0.5R - 0.9R; The horizontal inclination angle of the stirring paddle of the flat paddle stirrer is 10 - 20 degrees.

3. The method according to claim 2, wherein, The horizontal inclination angle of the stirring paddle of the flat paddle stirrer is 15 degrees.

4. The method according to claim 2, wherein The flat paddle stirrer is a four-blade flat paddle stirrer.

5. The method according to claim 4, wherein, The single-blade length of the paddle of the four-blade flat paddle stirrer is 0.9R.

6. The method according to claim 2, wherein During each stage of crystallization, the stirring speed is independently 20 - 65 revolutions per minute.

7. The method according to claim 3, wherein In the first-stage crystallization, the stirring speed is 35 - 60 revolutions per minute; In the second-stage crystallization, the stirring speed is 35 - 45 revolutions per minute; In the third-stage crystallization, the stirring speed is 30 - 40 revolutions per minute; In the fourth-stage crystallization, the stirring speed is 20 - 30 revolutions per minute.

8. The method according to claim 7, wherein In the fourth-stage crystallization, the stirring speed is 25 - 30 revolutions per minute.

9. The method according to claim 1, wherein The method is carried out in a crystallization system including at least four serially-connected crystallization tanks in sequence. The tank bodies of each stage of crystallization tank are not connected to each other. The raw material is fed into the crystallization tank for the first-stage crystallization and passes through each stage of crystallization tank in sequence.

10. The method according to claim 9, wherein, The temperature control method for each temperature zone in each stage of crystallization tank includes independently controlling the temperature through circulating water; The circulating water reaches the required temperature by adding cold water.

11. The method according to claim 1, wherein, The method further includes adding gluconolactone crystal seeds when the temperature of the raw material is 65 - 75 °C; 2 - 4 g of gluconolactone crystal seeds are added per kilogram of gluconic acid in the raw material.

12. The method according to claim 1, wherein The purity of the raw material is above 95%; The concentration of the raw material is 800 - 960 g / kg.

13. The method according to claim 12, wherein, The purity of the raw material is above 98%.

14. The method according to any one of claims 9 - 13, wherein, When the 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 starts; When the liquid in the crystallization process accounts for 85 ± 5% of the volume of the crystallization tank for the third-stage crystallization, the third-stage crystallization starts; When the liquid in the crystallization process accounts for 85 ± 5% of the volume of the crystallization tank for the fourth-stage crystallization, the fourth-stage crystallization starts.

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