Preparation method of superfine calcium citrate suitable for liquid calcium

Abstract

The invention discloses a preparation method of superfine calcium citrate suitable for liquid calcium, which comprises the following steps: S1, taking citric acid, citrate and calcium carbonate to react in water to obtain calcium citrate; the reaction temperature ranges from 80 DEG C to 90 DEG C, and the reaction time ranges from 10 min to 20 min; s2, performing vacuum freeze drying and primary crushing on the calcium citrate to obtain coarse powder; s3, absolute ethyl alcohol is added into the coarse powder and mixed evenly, low-temperature superfine grinding is carried out, the adding amount of the absolute ethyl alcohol is 8%-10% of the mass of the coarse powder, and the low-temperature superfine grinding temperature is 0-5 DEG C. On the basis of a normal calcium citrate preparation method, the calcium citrate is crushed and dispersed at a low temperature by using vacuum freeze drying, stabilizer adding and ice crystallization superfine grinding technologies, so that the structural stability of the calcium citrate is improved, and the problems of layering, thickening, solidification and the like are avoided when the calcium citrate is applied to liquid calcium.

Description

Technical Field

[0001] The present invention belongs to the technical field of food processing, and particularly relates to a preparation method of ultrafine calcium citrate applicable to liquid calcium. Background Art

[0002] As a new calcium supplement preparation at home and abroad in recent years, liquid calcium is usually prepared by ultra-low temperature freezing extraction and concentration to make the calcium into an emulsion state. The calcium inside is fine, with good fluidity and delicate taste. This special liquid state makes its specific surface area larger and easier to be absorbed by the human body, and at the same time, it will not cause calcium deposition and form stone symptoms in the human body.

[0003] The calcium sources of liquid calcium are diverse. The liquid calcium formula with inorganic calcium carbonate as the calcium source is more common, while the promotion of liquid calcium prepared with organic calcium citrate has been limited due to the existence of systematic problems such as stratification and solidification. Although the patents "Preparation Method of Nano Calcium Citrate" (Patent No. CN103755552A) and "A Preparation Method of Ultrafine Calcium Citrate" (Patent No. CN108218694A) both prepared calcium citrate products with extremely fine particle size from the synthesis end, there will still be problems such as solidification and stratification when applied in liquid calcium. Therefore, it is necessary to prepare a calcium citrate product applicable to the liquid calcium dosage form, increase the promotion and application of calcium citrate in the liquid calcium industry, and give full play to its advantages as a second-generation nutritional supplement with high absorption, high compatibility, and low irritation of organic calcium salts. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of ultrafine calcium citrate applicable to liquid calcium to solve the deficiencies of the prior art.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] A preparation method of ultrafine calcium citrate applicable to liquid calcium includes the following steps:

[0007] S1. Take citric acid, citrate and calcium carbonate and react them in water to obtain calcium citrate; the reaction temperature is 80 - 90 °C, and the reaction time is 10 - 20 min;

[0008] S2. Vacuum freeze-dry and perform primary crushing on the calcium citrate to obtain a coarse powder;

[0009] S3. Then add absolute ethanol to the coarse powder and mix evenly, and perform low-temperature ultrafine crushing. The addition amount of absolute ethanol is 8% - 10% of the mass of the coarse powder, and the low-temperature ultrafine crushing temperature is 0 - 5 °C.

[0010] Preferably, the amount of the citrate, the citric acid and the calcium carbonate is used in a molar ratio of (0.06-0.08):1:(1.4-1.6).

[0011] Preferably, step S1 further comprises:

[0012] Citric acid is added to 2 to 4 times the mass of water, the temperature is raised to 60 to 70° C., calcium carbonate is slowly added, the temperature is further raised to 80 to 90° C., citrate is then added, the temperature is kept to react for 10 to 15 minutes, and then filtered to obtain the calcium citrate.

[0013] Preferably, the vacuum freeze-drying temperature in step S2 is -20 to -40°C, and the time is 12 to 15 hours.

[0014] Preferably, after the primary crushing in step S2, the product is sieved through a 40-100 mesh sieve to obtain a coarse powder.

[0015] Preferably, the ultrafine grinding in step S3 is performed to an average particle size of 7 to 15 μm.

[0016] Preferably, the low-temperature ultrafine grinding in step S3 adopts any one of a low-temperature impact mill, a low-temperature vibration ultrafine grinder or a low-temperature airflow ultrafine grinder.

[0017] Preferably, the citrate in step S1 is selected from at least one of sodium citrate and potassium citrate.

[0018] Compared with the prior art, the preparation method provided in this application has the following beneficial effects:

[0019] 1. Calcium citrate wet material is prepared using basic water, citric acid, citrate and calcium carbonate as raw materials. Free water in the calcium citrate is removed by vacuum freeze drying, thereby maintaining the crystal form of the calcium citrate and improving its dispersibility.

[0020] 2. The semi-finished calcium citrate product is crushed by graded grinding, and then sent to the ultrafine grinder for fine processing after primary crushing to ensure the normal distribution of the final product particle size;

[0021] 3. Adding a small amount of anhydrous ethanol as a stabilizer and ice crystal dispersant before ice crystallization and low-temperature ultrafine grinding reduces the problem of product stickiness and clumping caused by static friction during ultrafine grinding and improves the efficiency of ice crystal grinding;

[0022] 4. The ice crystallization low-temperature ultrafine grinding technology is used to grind and disperse under low-temperature conditions, which reduces the agglomeration free energy of micron-sized products and improves their application effect. DETAILED DESCRIPTION

[0023] Although the calcium citrate products synthesized by chemical engineering can reach the micron or even nanometer level in terms of particle size, due to the polymerization of their chemical structure, obvious stratification and solidification phenomena still occur when they are applied in liquid calcium, which limits the application of calcium citrate in liquid calcium dosage forms. Based on the normal preparation method of calcium citrate, this application uses vacuum freeze-drying, adding stabilizers, and the technology of ice crystallization ultrafine pulverization to pulverize and disperse calcium citrate at low temperature, improving the stability of its structure and ensuring that there will be no problems such as stratification, thickening, and solidification when it is applied in liquid calcium. The detailed description is as follows:

[0024] The present invention provides a preparation method of ultrafine calcium citrate applicable to liquid calcium, comprising the following steps:

[0025] S1. Take citric acid, citrate, and calcium carbonate to react in water to obtain calcium citrate; the reaction temperature is 80-90 °C, the reaction time is 10-20 min, and the citrate added in the reaction serves as a pH buffer and an anion supplier; through experimental research, it is found that the reaction temperature and reaction time have obvious effects on the formation of calcium citrate. When the reaction temperature is low, it is easy to cause incomplete reaction of calcium carbonate, and needle-like solids are likely to precipitate when applied in liquid calcium; when the reaction temperature is too high, not only the energy consumption increases, but also calcium citrate is slightly sticky, the particle size is uneven after pulverization, and stratification will occur during application; when the reaction time is short, incomplete reaction is also likely to occur, and when the reaction time is too long, the product will be seriously sticky, the particle size of the product is unstable after pulverization, greatly deviating from the control range, and stratification problems will also occur in the product during application.

[0026] Preferably, the citrate is selected from at least one of sodium citrate and potassium citrate.

[0027] Preferably, the dosage of citrate, citric acid, and calcium carbonate is (0.06-0.08):1:(1.4-1.6) in terms of molar ratio, which can make the reaction of calcium carbonate complete.

[0028] Preferably, step S1 further includes:

[0029] Take citric acid and add 2-4 times the mass of water, heat up to 60-70 °C, slowly add calcium carbonate, continue to heat up to 80-90 °C, then add citrate thereto, keep the temperature for reaction for 10-15 min, and then filter to remove the reaction solution to obtain calcium citrate.

[0030] S2. Vacuum freeze-dry calcium citrate to remove the free water in calcium citrate, and then crush and sieve it to obtain coarse powder. Vacuum freeze-drying calcium citrate is beneficial to maintaining the crystal form of calcium citrate, improving its dispersibility. Primary crushing before ultrafine crushing can improve the ultrafine crushing effect, ensure that the particle size of the product meets the requirements, conforms to the normal distribution law, and reduces the layering of liquid calcium.

[0031] Preferably, the vacuum freeze-drying temperature is -20 to -40 °C, and the time is 12 to 15 h.

[0032] Preferably, after primary crushing, sieve through a 40-100 mesh sieve, take the material under the sieve for low-temperature ultrafine crushing, and re-crush the material on the sieve.

[0033] S3. Then add absolute ethanol to the coarse powder and mix evenly, and perform low-temperature ultrafine crushing. Preferably, the ultrafine crushing is carried out until the average particle size is 7-15 μm, the addition amount of absolute ethanol is 8%-10% of the mass of the coarse powder, and the low-temperature ultrafine crushing temperature is 0-5 °C.

[0034] Adding a small amount of absolute ethanol as a stabilizer and ice crystallization dispersant before low-temperature ultrafine crushing can reduce the problem of product sticking and agglomeration caused by electrostatic friction during ultrafine crushing, and improve the efficiency of ice crystallization crushing; then perform low-temperature ice crystallization ultrafine crushing, crush and disperse under low-temperature conditions, reduce the agglomeration free energy of micron-sized products, and improve their application effects.

[0035] Low-temperature ultrafine crushing can be carried out using a low-temperature impact mill, a low-temperature vibration ultrafine mill, a low-temperature air-flow ultrafine mill, etc.

[0036] The preparation method provided by this application has the following beneficial effects compared with the prior art:

[0037] 1. Using basic water, citric acid, citrate and calcium carbonate as raw materials to prepare wet calcium citrate, and removing the free water in calcium citrate by vacuum freeze-drying, maintaining the crystal form of calcium citrate and improving its dispersibility;

[0038] 2. Using graded crushing for the calcium citrate semi-finished product, and then entering an ultrafine mill for fine processing after primary crushing to ensure the normal distribution of the final product particle size;

[0039] 3. Adding a small amount of absolute ethanol as a stabilizer and ice crystallization dispersant before ice crystallization low-temperature ultrafine crushing, reducing the problem of product sticking and agglomeration caused by electrostatic friction during ultrafine crushing, and improving the efficiency of ice crystallization crushing;

[0040] 4. Using ice crystallization low-temperature ultrafine crushing technology, crushing and dispersing under low-temperature conditions, reducing the agglomeration free energy of micron-sized products, and improving their application effects.

[0041] Example 1

[0042] 100 g of citric acid monohydrate was added to 300 mL of water, and the temperature was raised to 65 °C. 70 g of calcium carbonate was slowly added thereto, and the temperature was further raised to 85 °C. 10 g of sodium citrate dihydrate was added thereto, and the mixture was kept at a constant temperature for reaction for 15 min, followed by filtration. The product was freeze-dried in vacuo at -30 °C for 14 h, and then the product was pulverized using a pulverizer and passed through a 60-mesh sieve. 10 g of absolute ethanol was sprayed onto the sieved product, and the product was put into a sterilized low-temperature pulsed ultramicro pulverizer. The temperature parameter was set at 2 °C, and low-temperature ultramicro pulverization was carried out. The average particle size of the prepared product was 8.135 μm, the mass percentage of calcium citrate was 98.68% (based on dry basis), the drying loss was 12.26%, and when applied to liquid calcium, the system was stable after accelerated storage for 90 days, and there were no problems of delamination, thickening and solidification.

[0043] Comparative Example 1 (lowering the reaction system temperature)

[0044] 100 g of citric acid monohydrate was added to 300 mL of water, and the temperature was raised to 65 °C. 70 g of calcium carbonate was slowly added thereto, and the mixture was kept at a constant temperature for reaction for 5 min. Then 10 g of sodium citrate dihydrate was added thereto, and the mixture was further kept at a constant temperature for reaction for 15 min, followed by filtration. The product was freeze-dried in vacuo at -30 °C for 14 h, and then the product was pulverized using a pulverizer and passed through a 60-mesh sieve. 10 g of absolute ethanol was sprayed onto the sieved product, and the product was put into a sterilized low-temperature pulsed ultramicro pulverizer. The temperature parameter was set at 2 °C, and low-temperature ultramicro pulverization was carried out. The average particle size of the prepared product was 12.448 μm, the mass percentage of calcium citrate was 99.27% (the calcium citrate content was obtained by converting the titrated calcium content, and this content was higher than that in Example 1. It was speculated that there was residual calcium carbonate in the product, resulting in a higher calcium citrate data), the drying loss was 11.84%, and when applied to liquid calcium, a small amount of needle-like crystals appeared at the bottom after accelerated storage for 90 days.

[0045] Comparative Example 2 (prolonging the reaction time)

[0046] 100 g of citric acid monohydrate was added to 300 mL of water, and the temperature was raised to 65 °C. 70 g of calcium carbonate was slowly added thereto, and the temperature was further raised to 85 °C. 10 g of sodium citrate dihydrate was added thereto, and the mixture was kept at a constant temperature for reaction for 60 min, followed by filtration. The product was freeze-dried in vacuo at -30 °C for 14 h, and then the product was pulverized using a pulverizer and passed through a 60-mesh sieve. 10 g of absolute ethanol was sprayed onto the sieved product, and the product was put into a sterilized low-temperature pulsed ultramicro pulverizer. The temperature parameter was set at 2 °C, and low-temperature ultramicro pulverization was carried out. The average particle size of the prepared product was 17.104 μm, the content was 98.79%, the drying loss was 13.19%, and when applied to liquid calcium, obvious delamination occurred after accelerated storage for 90 days.

[0047] Comparative Example 3 (not using vacuum freeze-drying)

[0048] Take 100 g of citric acid monohydrate and add it to 300 mL of water. Heat the mixture to 65 °C, and slowly add 70 g of calcium carbonate to it. Then continue to heat to 85 °C, and add 10 g of sodium citrate dihydrate to it. Keep the temperature for reaction for 15 min, filter the product. The product is dried in an oven at 80 °C for 18 h, and then the product is crushed using a crusher and sieved through a 60-mesh sieve. Spray 10 g of absolute ethanol onto the sieved product, and put it into a sterilized low-temperature pulsed ultrafine crusher. Set the temperature parameter to 2 °C and carry out low-temperature ultrafine crushing. The average particle size of the prepared product is 10.068 μm, the content is 98.19%, the drying loss is 11.40%, and when applied in liquid calcium, slight delamination occurs after accelerated storage for 90 days.

[0049] Table 1 Influence of different experimental conditions on the content and application effect of the product

[0050] Number Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Calcium citrate content / % 98.68 99.27 98.79 98.19 Loss on drying / % 12.26 11.84 13.19 11.40 Product particle size / um 8.135 12.448 17.104 10.068 Application effect Good Crystallization at the bottom Product delamination Product delamination

[0051] Comparing with Table 1, it can be seen that although the indicators of the product can meet the requirements when the reaction temperature is reduced, when applied in liquid calcium, needle-like solids will precipitate at the bottom under accelerated conditions. It is speculated that this is due to the incomplete reaction of calcium carbonate and the precipitation of calcium formed in liquid calcium; while prolonging the reaction time will cause the product to become sticky, and the particle size of the product is unstable after crushing, deviating greatly from the control range, which will cause serious delamination problems of the product during application; changing the drying method is due to the temperature difference during drying and crushing, which makes the crystal form of the product unstable and has a certain impact during application.

[0052] Comparative Example 4 (without primary crushing)

[0053] Take 100 g of citric acid monohydrate and add it to 300 mL of water. Heat the mixture to 65 °C, and slowly add 70 g of calcium carbonate to it. Then continue to heat to 85 °C, and add 10 g of sodium citrate dihydrate to it. Keep the temperature for reaction for 15 min, filter the product. The product is vacuum freeze-dried at -30 °C for 14 h, and then spray 10 g of absolute ethanol onto the product, and put it into a sterilized low-temperature pulsed ultrafine crusher. Set the temperature parameter to 2 °C and carry out low-temperature ultrafine crushing. The average particle size of the prepared product is 14.866 μm, the content is 99.12%, the drying loss is 12.31%, and when applied in liquid calcium, delamination occurs after accelerated storage for 90 days.

[0054] Comparative Example 5 (without adding stabilizer before cryogenic crystallization crushing)

[0055] 100 g of citric acid monohydrate was added to 300 mL of water, and the temperature was raised to 65 °C. 70 g of calcium carbonate was slowly added thereto, and the temperature was further raised to 85 °C. 10 g of sodium citrate dihydrate was added thereto, and the mixture was kept warm and reacted for 15 min. After filtration, the product was freeze-dried under vacuum at -30 °C for 14 h. Then the product was pulverized using a pulverizer and passed through a 60-mesh sieve. The sieved product was put into a sterilized low-temperature pulsed ultrafine pulverizer, and the temperature parameter was set at 2 °C for low-temperature ultrafine pulverization. The prepared product had an average particle size of 9.430 μm, a content of 98.51%, and a drying loss of 12.04%. When applied in liquid calcium, it became thick after being accelerated for 90 days.

[0056] Comparative Example 6 (without cryogenic crystallization pulverization)

[0057] 100 g of citric acid monohydrate was added to 300 mL of water, and the temperature was raised to 65 °C. 70 g of calcium carbonate was slowly added thereto, and the temperature was further raised to 85 °C. 10 g of sodium citrate dihydrate was added thereto, and the mixture was kept warm and reacted for 15 min. After filtration, the product was freeze-dried under vacuum at -30 °C for 14 h. Then the product was pulverized using a pulverizer and passed through a 60-mesh sieve. 10 g of absolute ethanol was sprayed onto the sieved product, and it was put into a sterilized pulsed ultrafine pulverizer for normal-temperature ultrafine pulverization at about 25 °C. The prepared product had an average particle size of 8.891 μm, a content of 98.92%, and a drying loss of 12.10%. When applied in liquid calcium, it solidified after being accelerated for 90 days.

[0058] Table 2 Influence of different pulverization techniques on product content and application effect

[0059] Number Example 1 Comparative Example 4 Comparative Example 5 Comparative Example 6 Calcium citrate content / % 98.68 99.12 98.51 98.92 Loss on drying / % 12.26 12.31 12.04 12.10 Product particle size / um 8.135 14.866 9.430 8.891 Application effect Good Product delamination Product thickening Product solidification

[0060] Comparing with Table 2, without coarse powder, there were certain fluctuations in the particle size of the product. Although it was still within the control range, there was a slight layering phenomenon in the liquid calcium after application; without adding a stabilizer before cryogenic crystallization pulverization, although the indicators of the pulverized product were normal, it became thick after application. It was speculated that the polymerization of calcium citrate decreased the stability of the product, resulting in the thickening problem; without using the cryogenic crystallization technique, the indicators of the product after ultrafine pulverization were not much different from those of Example 1, but obvious solidification occurred in the liquid calcium after application, and the whole system completely solidified and did not flow when inverted. This was because the agglomeration energy on the surface of calcium citrate increased sharply after ultrafine pulverization and re-aggregated in the form of van der Waals force after being briefly dispersed in the liquid calcium, causing the liquid calcium system to solidify.

[0061] Example 2

[0062] Take 100 g of citric acid monohydrate and add it to 300 mL of water. Heat the mixture to 60 °C, and slowly add 70 g of calcium carbonate to it. Then continue to heat it to 80 °C, and add 10 g of sodium citrate dihydrate to it. Keep the reaction at this temperature for 20 min, then filter. The product is vacuum freeze-dried at -30 °C for 14 h. After that, use a pulverizer to crush the product and sieve it through a 60-mesh sieve. Spray 10 g of absolute ethanol onto the sieved product, and put it into a sterilized low-temperature pulsed ultramicro pulverizer. Set the temperature parameter to 5 °C and carry out low-temperature ultramicro pulverization. The prepared product has an average particle size of 9.457 μm, a content of 98.13%, and a drying loss of 12.71%. When applied to liquid calcium and accelerated storage for 90 days, the system is stable without problems of stratification, thickening, and solidification.

[0063] Example 3

[0064] Take 100 g of citric acid monohydrate and add it to 300 mL of water. Heat the mixture to 70 °C, and slowly add 70 g of calcium carbonate to it. Then continue to heat it to 90 °C, and add 10 g of sodium citrate dihydrate to it. Keep the reaction at this temperature for 10 min, then filter. The product is vacuum freeze-dried at -30 °C for 14 h. After that, use a pulverizer to crush the product and sieve it through a 60-mesh sieve. Spray 10 g of absolute ethanol onto the sieved product, and put it into a sterilized low-temperature pulsed ultramicro pulverizer. Set the temperature parameter to 2 °C and carry out low-temperature ultramicro pulverization. The prepared product has an average particle size of 7.830 μm, a content of 97.97%, and a drying loss of 13.10%. When applied to liquid calcium and accelerated storage for 90 days, the system is stable without problems of stratification, thickening, and solidification.

[0065] Example 4

[0066] Take 100 g of citric acid monohydrate and add it to 300 mL of water. Heat the mixture to 65 °C, and slowly add 70 g of calcium carbonate to it. Then continue to heat it to 85 °C, and add 10 g of sodium citrate dihydrate to it. Keep the reaction at this temperature for 15 min, then filter. The product is vacuum freeze-dried at -30 °C for 14 h. After that, use a pulverizer to crush the product and sieve it through a 60-mesh sieve. Spray 10 g of absolute ethanol onto the sieved product, and put it into a sterilized low-temperature pulsed ultramicro pulverizer. Set the temperature parameter to 1 °C and carry out low-temperature ultramicro pulverization. The prepared product has an average particle size of 7.922 μm, a content of 98.83%, and a drying loss of 12.63%. When applied to liquid calcium and accelerated storage for 90 days, the system is stable without problems of stratification, thickening, and solidification.

[0067] Example 5

[0068] 100 g of citric acid monohydrate was added to 300 mL of water, and the temperature was raised to 65 °C. 68 g of calcium carbonate was slowly added thereto, and the temperature was further raised to 80 °C. 11 g of sodium citrate dihydrate was added thereto, and the mixture was kept at a constant temperature and reacted for 15 min. After filtration, the product was vacuum freeze-dried at -30 °C for 14 h. Then, the product was pulverized using a pulverizer and passed through an 80-mesh sieve. 11 g of absolute ethanol was sprayed onto the sieved product, and the product was put into a sterilized low-temperature pulsed ultrafine pulverizer. The temperature parameter was set at 2 °C, and low-temperature ultrafine pulverization was carried out. The average particle size of the prepared product was 9.104 μm, the content was 98.27%, and the drying loss was 11.85%. When applied to liquid calcium, the system was stable after accelerated storage for 90 days, and there were no problems of stratification, thickening, and solidification.

[0069] Example 6

[0070] 100 g of citric acid monohydrate was added to 300 mL of water, and the temperature was raised to 75 °C. 71 g of calcium carbonate was slowly added thereto, and the temperature was further raised to 90 °C. 9 g of sodium citrate dihydrate was added thereto, and the mixture was kept at a constant temperature and reacted for 10 min. After filtration, the product was vacuum freeze-dried at -20 °C for 15 h. Then, the product was pulverized using a pulverizer and passed through a 40-mesh sieve. 9.5 g of absolute ethanol was sprayed onto the sieved product, and the product was put into a sterilized low-temperature pulsed ultrafine pulverizer. The temperature parameter was set at 1 °C, and low-temperature ultrafine pulverization was carried out. The average particle size of the prepared product was 8.536 μm, the content was 99.10%, and the drying loss was 12.49%. When applied to liquid calcium, the system was stable after accelerated storage for 90 days, and there were no problems of stratification, thickening, and solidification.

[0071] Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications once they learn the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present invention. Obviously, those skilled in the art can make various changes and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A preparation method of ultrafine calcium citrate applicable to liquid calcium, characterized in that, It includes the following steps: S1. React citric acid, citrate and calcium carbonate in water to obtain calcium citrate; the reaction temperature is 80 - 90 °C, and the reaction time is 10 - 20 min; S2. Vacuum freeze-dry and perform primary pulverization on the calcium citrate to obtain a coarse powder; S3. Then add absolute ethanol to the coarse powder and mix evenly, and perform low-temperature ultrafine pulverization. The addition amount of absolute ethanol is 8% - 10% of the mass of the coarse powder, and the low-temperature ultrafine pulverization temperature is 0 - 5 °C.

2. The preparation method of ultrafine calcium citrate applicable to liquid calcium according to claim 1, characterized in that The dosage of the citrate, the citric acid and the calcium carbonate is (0.06 - 0.08):1:(1.4 - 1.6) in terms of molar ratio.

3. The preparation method of ultrafine calcium citrate applicable to liquid calcium according to claim 1, characterized in that Step S1 further includes: Take citric acid and add it to 2 - 4 times the mass of water, heat up to 60 - 70 °C, slowly add calcium carbonate, continue to heat up to 80 - 90 °C, then add citrate thereto, keep the temperature for reaction for 10 - 15 min, and then filter to obtain the calcium citrate.

4. The preparation method of ultrafine calcium citrate applicable to liquid calcium according to claim 1, characterized in that The vacuum freeze-drying temperature in step S2 is -20 - -40 °C, and the time is 12 - 15 h.

5. The preparation method of ultrafine calcium citrate applicable to liquid calcium according to claim 1, characterized in that After the primary pulverization in step S2, sieve through a 40 - 100 mesh sieve to obtain a coarse powder.

6. The preparation method of ultrafine calcium citrate applicable to liquid calcium according to claim 1, characterized in that The ultrafine pulverization in step S3 is to an average particle size of 7 - 15 μm.

7. The preparation method of ultrafine calcium citrate applicable to liquid calcium according to claim 1, characterized in that The low-temperature ultrafine pulverization in step S3 uses any one of a low-temperature impact mill, a low-temperature vibrating ultrafine pulverizer or a low-temperature air-flow ultrafine pulverizer.

8. The preparation method of ultrafine calcium citrate applicable to liquid calcium according to claim 1, characterized in that The citrate in step S1 is selected from at least one of sodium citrate and potassium citrate.