Preparation method of sodium citrate

Abstract

The invention provides a preparation method of sodium citrate, and relates to the technical field of preparation of sodium citrate. The method comprises the following steps: 1, primarily filtering citric acid fermentation clear liquid, mixing and stirring with a composite extraction phase, standing, extracting citric acid into the composite extraction phase to obtain an oil phase and a water phase, and filtering and washing the oil phase; 2, mixing and stirring the washed oil phase and a sodium hydroxide solution, standing, filtering, taking a water phase, cooling, crystallizing, centrifugally separating and washing to obtain sodium citrate; the composite extraction phase comprises an ionic liquid and an organic solvent. According to the preparation method, by optimizing the technological process, the common problem of waste water, waste gas and waste residue emission in a traditional method is solved, particularly, the risk of secondary pollution caused by using calcium carbonate, sulfuric acid and other materials is avoided, and meanwhile, the method is beneficial to reducing energy consumption.

Description

Technical Field

[0001] The invention relates to the technical field of sodium citrate preparation, in particular to a method for preparing sodium citrate. Background Art

[0002] As an indispensable food additive, sodium citrate is widely used in the food industry, pharmaceutical industry, cosmetics and other fields. Its use in these fields not only enhances the stability of the product, but also improves the taste, prolongs the shelf life, and plays an important role as a buffer and chelating agent. The traditional preparation method of sodium citrate usually starts with the fermentation production of citric acid, which is a biochemical process that converts sugars into citric acid through the action of specific microorganisms. After that, in order to obtain pure citric acid for subsequent reactions, a series of complex separation and purification steps are required, including filtration, ion exchange, concentration and other processes to remove impurities. Next, sodium citrate is generated by neutralization reaction of purified citric acid with sodium carbonate or sodium hydroxide. The last step is to convert the liquid form of sodium citrate into a solid product through crystallization and drying, which is easy to store and transport.

[0003] However, in practical applications, traditional methods face many challenges. First, the traditional process is long and complex, involving multiple stages of operation, which results in a long production cycle and increased time and resource costs. Second, the problem of product purity has always plagued manufacturers; despite the adoption of a variety of purification methods, it is still difficult to completely eliminate all impurities, affecting the quality of the final product.

[0004] Therefore, it is urgent to develop a new method for preparing sodium citrate to efficiently prepare high-quality sodium citrate. Summary of the invention

[0005] In view of this, the present invention provides a method for preparing sodium citrate with simple operation and high product purity, so as to improve the quality of the final product.

[0006] The present invention provides a method for preparing sodium citrate, comprising the following steps: Step 1: After the citric acid fermentation clear liquid is initially filtered, it is mixed and stirred with the composite extraction phase, and allowed to stand to allow the citric acid to be extracted into the composite extraction phase to obtain an oil phase and an aqueous phase, and the oil phase is filtered and washed; Step 2, mixing the oil phase washed in step 1 with a sodium hydroxide solution, letting it stand and then filtering, taking the water phase, cooling and crystallizing, centrifuging and washing, and obtaining sodium citrate; The composite extraction phase comprises an ionic liquid and an organic solvent.

[0007] In one or some possible embodiments, the ionic liquid is selected from one or more of imidazole ionic liquids, quaternary ammonium salt ionic liquids or pyridine ionic liquids; the organic solvent is selected from one or more of diethyl ether, butyl acetate or n-butanol.

[0008] Furthermore, the ionic liquid is a molten salt of an organic cation and an inorganic / organic anion, the organic cation is selected from one of 1-alkyl-3-methylimidazole cations, N-alkylpyridinium cations or tetraalkylammonium cations; the anion is selected from one of chloride ion, nitrate, acetate, tetrafluoroborate, hexafluorophosphate or bistrifluoromethanesulfonamide.

[0009] In one or some possible embodiments, the volume ratio of the ionic liquid to the organic solvent in the composite extraction phase is 1:1-10.

[0010] In one or some possible embodiments, the composite extraction phase further includes tributyl phosphate accounting for 0.5-2% of the total volume of the composite extraction phase.

[0011] In one or some possible embodiments, in step 1, the volume ratio of the citric acid fermentation supernatant to the composite extraction phase is 1:1-2; and the content of citric acid in the citric acid fermentation supernatant is 150-200 g / L.

[0012] In one or some possible embodiments, in step 1, the extraction conditions include: a temperature of 25-40° C. and a pH value of 2-4.

[0013] In one or some possible embodiments, the concentration of the sodium hydroxide solution is 20-40 g / 100 mL.

[0014] In one or some possible embodiments, in step 2, the mixing temperature is 55-75°C.

[0015] In one or some possible embodiments, in step 2, the cooling procedure is: cooling from the mixing temperature to 42-45° C. at a rate of 0.5-2° C. / min.

[0016] In one or some possible embodiments, in step 2, the volume ratio of the oil phase to the sodium hydroxide solution is 1:1-2.

[0017] The preparation method of sodium citrate provided by the present invention has the following beneficial effects compared with the prior art: (1) The present invention adopts a composite extraction phase of a specific composition and adds a trace amount of tributyl phosphate, which can efficiently and selectively extract citric acid from the citric acid fermentation supernatant. This unique composite extraction system not only reduces the interference of impurities and improves the recovery rate of citric acid, but also further optimizes the extraction efficiency by precisely controlling the extraction conditions. In the subsequent sodium hydroxide solution conversion process, strict temperature control and cooling rate ensure the complete conversion of citric acid to sodium citrate, reduce the possibility of by-product generation, and thus obtain high-purity, high-quality sodium citrate crystals.

[0018] (2) The method for preparing sodium citrate of the present invention reduces the wastewater, waste gas and waste residue emissions commonly seen in traditional methods by optimizing the process flow, and especially avoids the risk of secondary pollution caused by the use of materials such as calcium carbonate and sulfuric acid. On the other hand, reasonable temperature settings and an efficient extraction system help reduce energy consumption, thereby reducing long-term operating costs. DETAILED DESCRIPTION

[0019] The inventors have made the present invention through further research to improve the quality and production efficiency of sodium citrate products and simplify the production process. The present invention prepares sodium citrate products by the following steps, specifically including: Step 1: After the citric acid fermentation clear liquid is initially filtered, it is mixed and stirred with the composite extraction phase, and allowed to stand to allow the citric acid to be extracted into the composite extraction phase to obtain an oil phase and an aqueous phase, and the oil phase is filtered and washed; Step 2, mixing the oil phase washed in step 1 with a sodium hydroxide solution, letting it stand and then filtering, taking the water phase, cooling and crystallizing, centrifuging and washing, and obtaining sodium citrate; The composite extraction phase comprises an ionic liquid and an organic solvent.

[0020] In step one of the present invention, first, the citric acid fermentation clear liquid is pre-filtered, and the pre-filtration method can be selected from membrane filtration, plate pressure filtration or activated carbon, molecular sieve adsorption, the purpose is to remove larger solid impurities and insoluble substances, such as cell residues, protein clots and other suspended particles that may be present in the fermentation liquid, so as to avoid interfering with the subsequent extraction process and ensure that the selective separation effect of citric acid is not affected. Secondly, the citric acid fermentation clear liquid after the initial filtration is mixed and stirred with the composite extraction phase. The selective solubility characteristics of this composite extraction phase enable citric acid to be effectively transferred from the aqueous phase to the oil phase, thereby achieving efficient selective extraction; in this process, citric acid molecules form ion pairs with cations in the ionic liquid. Since citric acid has three carboxyl groups, it can form multiple hydrogen bonds or electrostatic interactions with cations in the ionic liquid, thereby enhancing its solubility in the organic phase. At the same time, stirring helps to improve the extraction rate of citric acid and ensure the quality of subsequent products. After standing, the mixture is naturally stratified to form an oil phase and an aqueous phase rich in citric acid. This process uses gravity to help citric acid molecules migrate to the organic phase more thoroughly, thereby achieving a better separation effect. This step is crucial to improve the purity of the final product and reduce the amount of citric acid remaining in the aqueous phase. Finally, by filtering and washing the oil phase, trace amounts of water and other impurities can be removed to ensure the high purity of the citric acid, while also preparing for the next step of the reaction and avoiding the impact of impurities on the sodium hydroxide back extraction in the subsequent step.

[0021] Furthermore, the content of citric acid in the citric acid fermentation supernatant is 150-200 g / L; the volume ratio of the citric acid fermentation supernatant to the composite extraction phase is 1:1-2. Under this condition, a higher citric acid concentration ensures sufficient effective ingredients in the raw material, improves the efficiency and economy of the subsequent extraction process, and reduces the additional cost and energy consumption caused by processing a large amount of diluent. At the same time, based on this volume ratio, the citric acid in the citric acid fermentation supernatant can fully contact with the extractant in the composite extraction phase, which helps to maximize the transfer of citric acid to the organic phase, improve the extraction efficiency, and ensure the effective separation of the target product.

[0022] Furthermore, the extraction conditions include: a temperature of 25 to 40°C and a pH of 2 to 4; first, within this temperature range, the solubility of citric acid in the aqueous phase can be ensured to be sufficiently high, and it is also beneficial for the composite extraction phase composed of ionic liquids and organic solvents to maintain good fluidity, thereby promoting the effective transfer of citric acid from the fermentation broth to the organic phase. Secondly, controlling the pH value between 2 and 4 can ensure that citric acid exists in molecular form, which is beneficial for its distribution coefficient between the two phases to reach the optimal state and improve the extraction efficiency. In addition, the appropriate pH can also prevent the decomposition or conversion of citric acid, ensure the maximum recovery rate of the target product, and reduce the co-extraction of impurities, which helps to improve the purity of the final product.

[0023] First, the oil phase washed in step 1 is fully mixed with the sodium hydroxide solution and stirred. This process can ensure that the citric acid in the oil phase is fully contacted with the sodium hydroxide and undergoes a neutralization reaction to generate sodium citrate, as shown in formula (I): C 6 H 8 O 7 +3NaOH→Na 3 C 6 H 5 O 7 +3H 2 O (I); This reaction releases water, and the generated sodium citrate has good water solubility, so that citric acid is transferred from the organic phase to the aqueous phase. After the mixture is left to stand for a period of time, the two phases are naturally separated to form an aqueous phase and an organic phase containing sodium citrate. This step not only helps to ensure that the citric acid is completely neutralized and avoids unreacted citric acid residues, but also promotes clear two-phase separation, which facilitates subsequent operations. Subsequently, by lowering the temperature, sodium citrate is crystallized from the solution to form a solid precipitate. Low temperature conditions are conducive to improving the crystallization efficiency of sodium citrate and reducing the co-crystallization phenomenon of impurities, thereby further improving the purity of the product. Then, the sodium citrate crystals are quickly and effectively separated from the solution by centrifugation. This step not only improves production efficiency, but also reduces the amount of mother liquor attached to the surface of the crystal. The crystals are then washed multiple times to remove residual moisture, unreacted sodium hydroxide and other possible impurities to ensure the high purity of the final product. Finally, the washed sodium citrate crystals are dried under vacuum drying conditions to prepare high-purity sodium citrate powder. This step can not only effectively remove residual moisture, but also avoid the impact of high temperature on product quality, ensuring the stability and quality of the final product.

[0024] Further, in step 2, the sodium hydroxide solution concentration is set to 20 ~ 40g / 100mL. Within this concentration range, sodium hydroxide can effectively react with citric acid to ensure that it is completely neutralized to generate sodium citrate, without causing incomplete reaction or requiring excessive solution due to low concentration, increasing the burden of subsequent concentration and separation. Secondly, the sodium hydroxide solution of appropriate concentration helps to control the reaction rate, avoids the violent exothermic reaction caused by excessive concentration, thereby reducing energy consumption and ensuring the safety of operation. In addition, this concentration range can also optimize the reaction conditions so that the solubility of sodium citrate in the solution is moderate, which is conducive to the subsequent efficient separation of pure sodium citrate crystals by the method of cooling crystallization, while reducing the co-crystallization of impurities, and improving the purity of the final product.

[0025] Further, in step 2, the volume ratio of the oil phase to the sodium hydroxide solution is 1:1-2. Under this condition, citric acid can be efficiently and completely transferred from the oil phase to the sodium hydroxide solution for neutralization reaction to generate sodium citrate. By controlling this volume ratio, incomplete extraction and waste of resources caused by excessive oil phase are avoided, and unnecessary alkaline byproducts and cost increase caused by excessive sodium hydroxide solution are also prevented. In addition, a reasonable volume ratio helps to form a stable emulsified system, promotes full contact and rapid mixing between the two phases, and improves the reaction rate and conversion efficiency.

[0026] In addition, it should be understood by those skilled in the art that the aqueous phase containing sodium citrate obtained after sufficient contact with the sodium hydroxide solution can be subjected to additional treatments such as decolorization, concentration, and drying according to actual operations. Conventional processes such as decolorization, concentration, and drying are common knowledge in the art and will not be described in detail herein.

[0027] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The present invention is further described below in conjunction with specific examples, and the protection scope of the present invention is not limited by the following examples. The materials mainly involved in the following examples are all conventional commercial products unless otherwise specified.

[0029] Table 1 Material source description

[0030] Example 1 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After filtering 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment), the pH value is adjusted to 3, and mixed with a composite extraction phase (trioctylmethylammonium chloride and n-butanol according to the v / v ratio of 1:2) and tributyl phosphate accounting for 1% of the total volume of the composite extraction phase at a volume ratio of 1:1 at 30°C, and stirred at 30°C for 10 minutes; standing and stratifying to extract citric acid into the composite extraction phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum to obtain about 406g of sodium citrate.

[0031] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 98.7% (dry basis), and the calculated yield was 98.2%.

[0032] Example 2 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After filtering 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment), the pH value is adjusted to 3, and mixed with a composite extract phase (1-butyl-3-methylimidazolium hexafluorophosphate and butyl acetate according to the v / v ratio of 1:2) and tributyl phosphate accounting for 1% of the total volume of the composite extract phase at a volume ratio of 1:1 at 30°C, and stirred at 30°C for 10 minutes; standing and stratifying to extract citric acid into the composite extract phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum to obtain about 409.3g of sodium citrate.

[0033] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.5% (dry basis), and the calculated yield was 99%.

[0034] Example 3 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment) is initially filtered, the pH value is adjusted to 3, and at 30° C., it is mixed with a composite extract phase (1-butylpyridine bistrifluoromethanesulfonyl imide salt and butyl acetate according to a v / v ratio of 1:2) and tributyl phosphate accounting for 1% of the total volume of the composite extract phase in a volume ratio of 1:1, and stirred at 30° C. for 10 minutes; standing and stratifying to extract citric acid into the composite extract phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand the stratification to transfer the sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum conditions to obtain about 405.1g of sodium citrate.

[0035] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 98.9% (dry basis), and the calculated yield was 98.0%.

[0036] Example 4 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment) is initially filtered, the pH value is adjusted to 3, and at 30° C., it is mixed with a composite extract phase (1-butyl-3-methylimidazolium hexafluorophosphate and butyl acetate according to a v / v ratio of 1:2) and tributyl phosphate accounting for 1% of the total volume of the composite extract phase in a volume ratio of 1:2, and stirred at 30° C. for 10 minutes; standing and stratifying to extract citric acid into the composite extract phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum to obtain about 408.4g of sodium citrate.

[0037] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.4% (dry basis), and the calculated yield was 98.8%.

[0038] Example 5 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment) is initially filtered, the pH value is adjusted to 3, and at 30° C., it is mixed with a composite extract phase (1-butyl-3-methylimidazolium hexafluorophosphate and butyl acetate according to a v / v ratio of 1:6) and tributyl phosphate accounting for 1% of the total volume of the composite extract phase in a volume ratio of 1:1, and stirred at 30° C. for 10 minutes; standing and stratifying to extract citric acid into the composite extract phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum to obtain about 407.7g of sodium citrate.

[0039] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.3% (dry basis), and the calculated yield was 98.6%.

[0040] Example 6 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After pre-filtration, 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment) is adjusted to a pH value of 3, and mixed with a composite extract phase (1-butyl-3-methylimidazolium hexafluorophosphate and butyl acetate according to a v / v ratio of 1:10) and tributyl phosphate accounting for 1% of the total volume of the composite extract phase at 30° C. in a volume ratio of 1:1, and stirred at 30° C. for 10 minutes; standing and stratifying to extract citric acid into the composite extract phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum to obtain about 407g of sodium citrate.

[0041] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.2% (dry basis), and the calculated yield was 98.4%.

[0042] Example 7 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After pre-filtration, 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment) is adjusted to a pH value of 3, and mixed with a composite extract phase (1-butyl-3-methylimidazolium hexafluorophosphate and butyl acetate according to a v / v ratio of 1:2) and tributyl phosphate accounting for 0.5% of the total volume of the composite extract phase at a volume ratio of 1:1 at 30°C, and stirred at 30°C for 10 minutes; standing and stratifying to extract citric acid into the composite extract phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum conditions to obtain about 407.2g of sodium citrate.

[0043] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.3% (dry basis), and the calculated yield was 98.5%.

[0044] Example 8 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment) is initially filtered, the pH value is adjusted to 3, and at 30° C., it is mixed with a composite extract phase (1-butyl-3-methylimidazolium hexafluorophosphate and butyl acetate according to a v / v ratio of 1:2) and tributyl phosphate accounting for 2% of the total volume of the composite extract phase in a volume ratio of 1:1, and stirred at 30° C. for 10 minutes; standing and stratifying to extract citric acid into the composite extract phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 35g / 100mL in a volume ratio of 1:1, and stir at 60°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 0.5°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum conditions to obtain about 409g of sodium citrate.

[0045] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.4% (dry basis), and the calculated yield was 98.9%.

[0046] Example 9 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After pre-filtration, 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment) is adjusted to a pH value of 4, and mixed with a composite extraction phase (trioctylmethylammonium chloride and n-butanol according to a v / v ratio of 1:2) and tributyl phosphate accounting for 1% of the total volume of the composite extraction phase at a volume ratio of 1:1 at 25°C, and stirred at 30°C for 10 minutes; standing and stratifying to extract citric acid into the composite extraction phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 20g / 100mL in a volume ratio of 1:2, and stir at 70°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 42°C at a rate of 1°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum to obtain about 405.6g of sodium citrate.

[0047] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.1% (dry basis), and the calculated yield was 98.1%.

[0048] Example 10 This embodiment provides a method for preparing sodium citrate, comprising the following steps: Step 1: After filtering 1.5 L of citric acid fermentation clear liquid (the initial citric acid content is taken as 180 g / L according to the average value, the same as in the following embodiment), the pH value is adjusted to 2, and mixed with a composite extraction phase (trioctylmethylammonium chloride and n-butanol according to the v / v ratio of 1:2) and tributyl phosphate accounting for 1% of the total volume of the composite extraction phase at a volume ratio of 1:1 at 40°C, and stirred at 30°C for 10 minutes; standing and stratifying to extract citric acid into the composite extraction phase, to obtain an oil phase and an aqueous phase rich in citric acid, filtering and washing the oil phase with deionized water at least three times; Step 2: Mix the oil phase washed in step 1 with a sodium hydroxide solution having a concentration of 40g / 100mL in a volume ratio of 1:1, and stir at 70°C for 30min to promote the full reaction of citric acid and sodium hydroxide to form sodium citrate. After the reaction is completed, stand for stratification to transfer sodium citrate to the aqueous phase. Filter the aqueous phase and immediately place it in an ice water bath, cool it to 45°C at a rate of 2°C / min, and keep it at this temperature for 2 hours to promote the crystallization of sodium citrate. Centrifuge the suspension containing sodium citrate crystals at 4000rpm for 15 minutes to collect sodium citrate crystals. Wash the sodium citrate crystals at least three times with deionized water, and dry the excess water on the surface of the crystals after each washing. Finally, dry overnight under vacuum to obtain about 407.7g of sodium citrate.

[0049] The sodium citrate prepared in this example was measured by EDTA titration, and the content of sodium citrate was measured to be 99.2% (dry basis), and the calculated yield was 98.6%.

[0050] Comparative Example 1 The difference from Example 2 is that tributyl phosphate is not added in step 1, and the other steps remain unchanged.

[0051] The sodium citrate prepared in this comparative example was determined by EDTA titration, and the content of sodium citrate was measured to be 95.9% (dry basis), and the yield was calculated to be 95%.

[0052] Comparative Example 2 The difference from Example 2 is that in step 1, only butyl acetate is used in a volume ratio of 1:1 to citric acid fermentation supernatant, and the other steps remain unchanged.

[0053] The sodium citrate prepared in this comparative example was measured by EDTA titration method, and the content of sodium citrate was measured to be 93.8% (dry basis), and the calculated yield was 93%.

[0054] Comparative Example 3 The difference from Example 2 is that in step 1, 3% tributyl phosphate is added to the total volume of the composite extract phase, and the other steps remain unchanged.

[0055] The sodium citrate prepared in this comparative example was measured by EDTA titration method, and the content of sodium citrate was measured to be 98.2% (dry basis), and the calculated yield was 96.5%.

[0056] Comparative Example 4 The difference from Example 8 is that in step 1, 1-butyl-3-methylimidazole hexafluorophosphate is used in a volume ratio of 1:1 to the citric acid fermentation supernatant, and tributyl phosphate is added accounting for 2% of the volume of the extract phase, and the other steps remain unchanged.

[0057] The sodium citrate prepared in this comparative example was measured by EDTA titration method, and the content of sodium citrate was measured to be 97.0% (dry basis), and the calculated yield was 95%.

[0058] Comparative Example 5 The difference from Example 8 is that in step 1, 1-butyl-3-methylimidazole hexafluorophosphate is used in a volume ratio of 1:1 to citric acid fermentation supernatant, no organic solvent and tributyl phosphate are added, and the other steps remain unchanged.

[0059] The sodium citrate prepared in this comparative example was measured by EDTA titration, and the content of sodium citrate was measured to be 92.7% (dry basis), and the yield was calculated to be 92%.

[0060] Comparative Example 6 The difference from Example 8 is that in step 2, the cooling rate is 0.2°C / min, and the other steps remain unchanged.

[0061] The sodium citrate prepared in this comparative example was determined by EDTA titration, and the content of sodium citrate was measured to be 99.5% (dry basis), and the yield was calculated to be 97%.

[0062] By comparing the test results of Examples 1 to 3, it can be seen that the composite extraction phase composed of 1-butyl-3-methylimidazolium hexafluorophosphate and butyl acetate in a volume ratio of 1:2 in Example 2 performs best. This is because 1-butyl-3-methylimidazolium hexafluorophosphate, as an imidazole ionic liquid, has a stronger π-π stacking effect in its molecular structure, and can form a more stable hydrogen bond network with the carboxyl group of citric acid, thereby improving the selectivity and extraction efficiency of citric acid. In contrast, the 1-butylpyridine bistrifluoromethanesulfonyl imide salt used in Example 3 has a poor polarity of the pyridine ring, which limits its effective interaction with citric acid, resulting in an extraction effect that is not as good as that of imidazole ionic liquids. The trioctylmethylammonium chloride used in Example 1 belongs to a quaternary ammonium salt compound. Although it also has certain surface activity and dispersibility, it lacks specific interaction with citric acid, so it has limited effect in promoting the transfer of citric acid from the aqueous phase to the organic phase. Therefore, the inventor further selects imidazole ionic liquids with organic solvents to achieve a more excellent extraction effect.

[0063] It can be seen from the test results of Examples 2 and 4 above that adjusting the ratio of the citric acid fermentation supernatant and the composite extraction phase is beneficial to improving the extraction efficiency, but will not have a significant impact on the final quality and yield of the product.

[0064] By comparing the test results of Example 2 and Examples 5-6, it can be seen that in the composite extraction phase, the organic solvent is beneficial to reduce the viscosity of the system and improve the extraction efficiency, but excessive use may dilute the concentration of the ionic liquid, reduce the contact probability between the ionic liquid and citric acid, and weaken its extraction ability for citric acid. Therefore, combined with cost-effectiveness considerations, the inventor further believes that when the volume ratio of the ionic liquid to the organic solvent is controlled between 1:1-2, better economic benefits can be achieved.

[0065] From the test results of the above-mentioned Example 2 and Examples 7-8, it can be seen that tributyl phosphate, as an auxiliary agent, can improve the interfacial tension, enhance the extraction effect through the P=O bond, and promote the separation of the two phases. However, in Comparative Example 1, tributyl phosphate was not added at all, and its extraction efficiency was low. This is because the lack of surfactant weakened the effect of the hydrogen bond network in the system, so that the two phases could not be well separated; at the same time, excessive tributyl phosphate was added in Comparative Example 3, which may cause the system to produce emulsification, resulting in a decrease in yield. In addition, in combination with Example 2 and Comparative Example 2, the use of imidazole ionic liquids can specifically bind to citric acid molecules, which can significantly improve the extraction selectivity compared to the single use of butyl acetate.

[0066] Combining the test results of Example 8 and Comparative Examples 4-5, it can be seen that: without the addition of butyl acetate, even if 1-butyl-3-methylimidazolium hexafluorophosphate can form a stable hydrogen bond network with citric acid through its unique chemical structure, the higher viscosity of the system will reduce the mass transfer efficiency and may introduce impurities into the crystals, thereby affecting the purity and yield of the final product. In addition, the extraction efficiency of ionic liquids alone will be greatly reduced, and the product performance will also be reduced. By comparing Example 8 and Comparative Example 6, it can be seen that although slow cooling is conducive to the formation of larger and pure crystals, the reduction in product yield and the extension of the production cycle will lead to an increase in production costs.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing sodium citrate, characterized in that: The method comprises the following steps: Step 1: After the citric acid fermentation clear liquid is initially filtered, it is mixed and stirred with the composite extraction phase, and allowed to stand to allow the citric acid to be extracted into the composite extraction phase to obtain an oil phase and an aqueous phase, and the oil phase is filtered and washed; Step 2, mixing the oil phase washed in step 1 with a sodium hydroxide solution, letting it stand and then filtering, taking the water phase, cooling and crystallizing, centrifuging and washing, and obtaining sodium citrate; The composite extraction phase comprises an ionic liquid and an organic solvent.

2. The method for preparing sodium citrate according to claim 1, wherein The ionic liquid is selected from one or more of imidazole ionic liquids, quaternary ammonium salt ionic liquids or pyridine ionic liquids; the organic solvent is selected from one or more of diethyl ether, butyl acetate or n-butanol.

3. The method for preparing sodium citrate according to claim 2, wherein The volume ratio of the ionic liquid to the organic solvent in the composite extraction phase is 1:1-10.

4. The method for preparing sodium citrate according to claim 3, wherein The composite extraction phase also includes tributyl phosphate accounting for 0.5-2% of the total volume of the composite extraction phase.

5. The method for preparing sodium citrate according to claim 1, characterized in that: In step 1, the volume ratio of the citric acid fermentation clear liquid to the composite extraction phase is 1:1-2.

6. The method for preparing sodium citrate according to claim 5, characterized in that: In step 1, the extraction conditions include: temperature of 25-40° C. and pH of 2-4.

7. The method for preparing sodium citrate according to claim 1, wherein In step 2, the concentration of the sodium hydroxide solution is 20-40 g / 100 mL.

8. The method for preparing sodium citrate according to claim 7, characterized in that: In step 2, the mixing temperature is 55-75°C.

9. The method for preparing sodium citrate according to claim 8, characterized in that: In step 2, the cooling procedure is: cooling from the mixing temperature to 42-45°C at a rate of 0.5-2°C / min.

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