A kind of purification preparation method of sodium citrate

Abstract

The invention belongs to the technical field of sodium citrate preparation, and specifically relates to a method for purifying and preparing sodium citrate. Citrus peel residue and shell are used as raw materials, and alkylammonium chloride is used as a phase transfer catalyst for double decomposition reaction, so that a faster reaction speed can be achieved at a lower temperature, and strontium element in the sodium citrate can be removed.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium citrate preparation, and specifically relates to a purification preparation method of sodium citrate. Background Art

[0002] Sodium citrate is an important citrate salt that can be used as a flavoring agent and stabilizer in the food and beverage industry; as an anticoagulant, expectorant and diuretic in the pharmaceutical industry; in the detergent industry, it can replace sodium tripolyphosphate as an auxiliary agent for non-toxic detergents; it is also widely used in brewing, injections, photographic chemicals and electroplating.

[0003] In the prior art, the preparation methods of sodium citrate are different due to the different sources of raw materials. The current mainstream preparation method of sodium citrate is the citric acid soda method, that is, neutralizing a citric acid solution with an alkali to generate a sodium citrate solution. Although this method has low cost and readily available raw materials, since the raw material uses a citric acid fermentation broth with complex components, which contains a variety of impurities such as microbial cells, residual sugar, inorganic salts, proteins, pigments and organic acids, a more complicated impurity removal process is required, resulting in a relatively complicated overall production process, especially since metal ions in the fermentation broth are easily combined with citric acid and are difficult to remove.

[0004] my country is one of the main origin countries of citrus fruits. Canned products are the main processing method for citrus fruits in my country. A large amount of peel residue will remain in the production process, of which the peel accounts for 20%-40% of the fresh fruit. The country produces more than 10 million tons of citrus peel residue each year. It is a pity to discard these waste materials. Using citrus peel residue as raw material to prepare citric acid is a more economical solution.

[0005] In addition, my country has abundant shell resources and a wide variety of species, and shells contain a large amount of calcium. Modern technology can easily extract calcium oxide from shells for the preparation of other products. After searching, the patent document with publication number CN104086396A discloses a shell-derived calcium citric acid complex product and preparation method, which uses shells as raw materials, uses acid solution, converts water-insoluble inorganic calcium salts into water-soluble calcium ions, and then separates solids and liquids to remove impurities. After the calcium ions are complexed and precipitated by substances containing citrate, they are processed into calcium citric acid complex products. The patent with publication number CN110256231A discloses a preparation method of nano calcium citrate using shells as raw materials, which acidifies waste shells and citric acid in appropriate proportions to generate calcium citrate, and adds an appropriate amount of surfactant to control its particle size and crystal growth rate, and makes a nanometer-sized, food-grade purity calcium citrate powder. The product prepared by this invention can be directly used as a food additive, functional health food, beverage and feed additive or even medicine. The above disclosed documents all provide methods for preparing calcium citrate from shells, and calcium citrate can be used to prepare sodium citrate.

[0006] Strontium (Sr) is one of the important trace elements in mollusk shells. In the shells of many shellfish, strontium is the second (after calcium) or third (after calcium and sodium) metal element in content. For example, in the shells of tetrapod clams, the calcium content is about 3.83×10 5 μg / g -1 , the sodium content is about 5.8×10 4 μg / g -1 , the strontium content is about 1.4×10 4 μg / g -1 , while the content of magnesium, iron and potassium is only 150-500μg / g -1 , the content of metal elements such as zinc and copper is less than 50μg / g -1 Therefore, the content of strontium in shells is much greater than other trace metal elements; and, because the chemical properties of strontium are similar to those of calcium, a certain amount of strontium is inevitably contained when shells are used as raw materials to prepare citric acid derivatives. The presence of strontium may affect the purity and performance of sodium citrate, especially in applications that require high-purity sodium citrate, such as food additives, medicine, and other fields. Summary of the invention

[0007] The present invention provides a method for purifying and preparing sodium citrate to solve the defects pointed out in the background technology.

[0008] According to a first aspect of an embodiment of the present invention, a method for purifying and preparing sodium citrate comprises the following steps:

[0009] Step 1: calcining and crushing the discarded shells to obtain shell powder;

[0010] Step 2: adding dilute hydrochloric acid to the shell powder to obtain a calcium chloride solution; heating the calcium chloride solution at 60° C. to 80° C. to evaporate water to obtain a saturated calcium chloride solution;

[0011] Step 3: adding citric acid and / or citric acid fermentation broth to the saturated calcium chloride solution to obtain calcium citrate precipitate;

[0012] Step 4: adding the calcium citrate precipitate to the sodium carbonate solution, adding a phase transfer catalyst, reacting for 40-120 minutes, and obtaining a crude sodium citrate solution through double decomposition reaction;

[0013] Step 5: filtering, extracting, concentrating, washing and drying the crude sodium citrate solution to obtain a sodium citrate product.

[0014] In one aspect of the embodiments of the present invention, the discarded shells are one or more of marine shells, aquatic shells, and terrestrial shells other than abalone shells. Specifically, the discarded shells are oyster shells, blue mussel shells, clam shells, tetrapod clam shells, or pear-shaped ring-shaped snails.

[0015] In one aspect of the embodiments of the present invention, the citric acid fermentation broth is prepared by the following steps:

[0016] Step 3a-1: crushing citrus peel residue to obtain citrus peel residue powder; activating brewer's yeast in a glucose solution to obtain activated yeast;

[0017] Step 3a-2: adding the citrus peel residue powder to water, and then adding the activated yeast, and fermenting and culturing at 25° C.-35° C.; during the fermentation process, measuring the sugar content of the fermentation liquid every day until the residual sugar content no longer changes, and then ending the fermentation;

[0018] Step 3a-3: Boil the fermentation liquid after fermentation for 3-8 minutes, stop heating, reduce the temperature of the fermentation liquid to below 70°C, filter, decolorize and centrifuge, and obtain the supernatant after centrifugation to obtain the citric acid fermentation liquid.

[0019] In one aspect of the embodiment of the present invention, in step 4, the concentration of the sodium carbonate solution is 8%-16%. Specifically, in step 4, the concentration of the sodium carbonate solution is 8%, 10%, 13% or 15%, but is not limited thereto.

[0020] In one aspect of the embodiment of the present invention, in step 4, the reaction temperature is 5° C.-80° C. Preferably, in step 4, the reaction temperature is 20° C.-30° C.

[0021] In one aspect of an embodiment of the present invention, the phase transfer catalyst is an alkylammonium chloride catalyst. Preferably, the phase transfer catalyst is dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, n-decyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, hexadecyltrimethylammonium chloride, tri-n-nonylmethylammonium chloride or tri-n-nonylethylammonium chloride, but is not limited thereto. Further preferably, the phase transfer catalyst is tri-n-nonylethylammonium chloride.

[0022] In one aspect of the embodiments of the present invention, the tri-n-nonylethylammonium chloride is prepared by the following steps:

[0023] Step 4a-1: Add tri-n-nonylamine to acetonitrile and continue stirring, then add ethyl bromide, react at 20°C-40°C for 4-12 hours, after the reaction is completed, mix the reacted solution with water, stir at 30°C-45°C for 5-20 minutes, and collect the oil phase after standing;

[0024] Step 4a-2: washing the collected oil phase with a sodium hydroxide solution, a sodium chloride solution and water respectively, and then reducing the pressure and draining to obtain tri-n-nonylethylammonium bromide;

[0025] Step 4a-3: Add the obtained tri-n-nonylethylammonium bromide to an organic solvent, then add A5 molecular sieve and silver chloride, and shake in a lightless environment for 4-12 hours. After the reaction is completed, filter, reduce the pressure and drain the solution to obtain tri-n-nonylethylammonium chloride.

[0026] According to a second aspect of an embodiment of the present invention, a sodium citrate is obtained by the aforementioned method for purifying and preparing sodium citrate.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The invention uses discarded citrus peel residues and shells as raw materials, which is low in cost and environmentally friendly.

[0029] Furthermore, by using an alkylammonium chloride phase transfer catalyst, the reaction temperature of the metathesis reaction can be greatly reduced; when no catalyst is added, the metathesis reaction is at an optimal reaction temperature of about 55°C, and the addition of the alkylammonium chloride catalyst allows it to have a higher yield at room temperature, thereby not only reducing energy consumption, but also reducing the use of reaction equipment in expanded production, thereby reducing costs. In addition, the present invention prepares a catalyst with good catalytic ability for the reaction of strontium citrate to generate sodium citrate. Under the catalytic action of the catalyst, strontium citrate can be reacted well to generate sodium citrate, so as to remove strontium in sodium citrate generated using shells as raw materials.

[0030] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. The relevant embodiments described herein are illustrative and are used to provide a basic understanding of the present invention. The embodiments of the present invention should not be interpreted as limiting the present invention.

[0032] For simplicity, only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, and any upper limit can be combined with any other upper limit to form an unspecified range. In addition, each separately disclosed point or single value can itself be combined as a lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an unspecified range.

[0033] In this article, the terms "comprises", "comprising" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device that includes a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of more restrictions, an element defined by the sentence "comprising a ..." does not exclude the presence of other identical elements in the process, method, article or device that includes the element.

[0034] In the description herein, unless otherwise specified, “above” and “below” include the number.

[0035] Unless otherwise specified, the terms used in the present invention have the commonly known meanings generally understood by those skilled in the art. Unless otherwise specified, the numerical values ​​of the various parameters mentioned in the present invention can be measured by various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the embodiments of the present invention).

[0036] The term "about" is used to describe and illustrate small changes. When used in conjunction with an event or situation, the term may refer to an example in which the event or situation occurs precisely and an example in which the event or situation occurs very approximately. For example, when used in conjunction with a numerical value, the term may refer to a range of variation less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. In addition, amounts, ratios, and other numerical values ​​are sometimes presented in this article in a range format. It should be understood that such a range format is for convenience and simplicity, and should be flexibly understood to include not only numerical values ​​explicitly designated as range limits, but also all individual numerical values ​​or subranges encompassed within the range, as if each numerical value and subrange were explicitly designated.

[0037] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0038] The present invention provides a method for purifying and preparing sodium citrate, comprising the following steps:

[0039] Step 1: calcining and crushing discarded shells to obtain shell powder; in step 1, the discarded shells are oyster shells, blue mussel shells, clam shells, quadrangular clam shells or pear-shaped ring-shaped snails; the shell powder obtained after calcining and crushing the above shells mainly includes CaO; the chemical reaction formula is: CaCO3→CaO+CO2↑.

[0040] Step 2: Add dilute hydrochloric acid to the shell powder to obtain a calcium chloride solution; heat the calcium chloride solution at 60°C-80°C to evaporate water to obtain a saturated calcium chloride solution; the chemical reaction formula is: CaO+2HCl→CaCl2+H2O. Similarly, the saturated calcium chloride solution also contains strontium chloride.

[0041] Step 3: Add citric acid fermentation broth (or directly add commercially available citric acid) to the saturated calcium chloride solution obtained in step 2 to obtain calcium citrate precipitate; the chemical reaction formula is: 3CaCl2+2C6H8O7+4H2O→Ca3(C6H5O7)2↓+6H2O+6NaCl. Similarly, the calcium citrate precipitate also contains strontium citrate.

[0042] In step 3, there is no specific limitation on the concentration and volume of the added citric acid solution, as long as the addition of the citric acid solution is stopped after no more precipitation occurs.

[0043] Step 4: Add the calcium citrate precipitate to the sodium carbonate solution, add a phase transfer catalyst at the same time, react at 20°C-30°C for 40-120min, and obtain a crude sodium citrate solution through double decomposition reaction; the chemical reaction formula is: Ca3(C6H5O7)2+3Na2CO3→2Na3(C6H5O7)+3CaCO3↓.

[0044] In the prior art, the reaction of calcium citrate and sodium carbonate to produce sodium citrate by double decomposition needs to be carried out under heating. However, since the reaction rate of strontium citrate and sodium carbonate is slower than that of calcium citrate and sodium carbonate, the slightly soluble strontium citrate is redissolved in the reaction solution under heating, resulting in a still high strontium content in the sodium citrate product. If the reaction time is prolonged, not only energy is wasted, but also the yield of the double decomposition reaction will decrease as time goes by.

[0045] Step 5: filtering, extracting, concentrating, washing and drying the crude sodium citrate solution to obtain a sodium citrate product.

[0046] The present invention also provides a method for preparing citric acid fermentation broth, comprising:

[0047] Step 3a-1: Weigh 35 g of citrus peel residue and crush it to obtain citrus peel residue powder; activate brewer's yeast in a glucose solution to obtain activated yeast.

[0048] The activation process of brewer's yeast includes the following steps: weigh 1g of yeast, add 20mL of 10% glucose solution, activate at 38°C for 1h, and shake every 20min to obtain activated yeast.

[0049] Step 3a-2: Add citrus peel residue powder to water, then add activated yeast (at a ratio of 8% of the mass of the fermentation liquid), mix well, cover the bottle mouth with 3 layers of gauze, and then place in an incubator at 30°C for fermentation. During the fermentation process, measure the sugar content of the fermentation liquid every day until the residual sugar content no longer changes, and then terminate the fermentation;

[0050] Step 3a-3: Boil the fermentation liquid after fermentation for 5 minutes, then stop heating, wait for the temperature of the fermentation liquid to drop below 70° C., filter, decolorize and centrifuge, and take the supernatant obtained after centrifugation to obtain citric acid fermentation liquid.

[0051] The purpose of heating is to expand and rupture the yeast hyphae, release the citric acid in the hyphae, and increase the citric acid content; high temperature will denature and coagulate the protein in the fermentation liquid, reduce the viscosity of the fermentation liquid, and speed up the filtration; high temperature will also kill various microorganisms in the fermentation liquid. The purpose of filtration is to remove large impurities such as mycelium and orange peel powder residue in the fermentation liquid. The decolorization step is to add 3% of the activated carbon of the fermentation liquid mass to the filtered fermentation liquid, slowly stir it in a water bath at 65°C to adsorb the pigments and macromolecular impurities in the filtrate, and after stirring the filtrate for 30 minutes, let it stand in the air and cool to room temperature; the centrifugal speed is 2000-5000r / min, and the centrifugal time is 3-15min.

[0052] The main components of the supernatant were detected by paper chromatography. The citric acid standard solution, the fermentation supernatant obtained by pretreatment of the fermentation broth, the oxalic acid standard solution and the malic acid standard solution were spot-sampled and analyzed on the same filter paper to determine the R f The main components of the fermentation liquid were determined by the value. The developing agent (volume ratio) was n-butanol: formic acid: water: glacial acetic acid = 4:1:4:1. The main component of the fermentation supernatant was citric acid as shown by paper chromatography.

[0053] The citric acid used in the present invention may also be commercially available citric acid, and the present invention does not impose any specific limitation on this.

[0054] The present invention also provides a method for preparing a tri-n-nonylethylammonium chloride catalyst, the solubility of the catalyst in an organic solvent and water is very different, and it is easy to remove by extraction after the reaction is completed. The method comprises:

[0055] Step 4a-1: Add 1 mol of tri-n-nonylamine to 100 mL of acetonitrile and continue stirring (150-500 r / min), then add 1 mol of ethyl bromide, and react in a 25°C water bath for 6 hours. After the reaction is completed, mix the reacted solution with 200 mL of water, stir at 40°C for 8 minutes, and collect the oil phase after standing;

[0056] Step 4a-2: The collected oil phase is washed with 300 mL of 0.15 mol / L sodium hydroxide solution, 300 mL of 1.2 mol / L sodium chloride solution and 300 mL of deionized water, respectively, and then decompressed and dried to obtain tri-n-nonylethylammonium bromide;

[0057] Step 4a-3: The obtained tri-n-nonylethylammonium bromide was added to 200 mL of propanol, and then 1 g of A5 molecular sieve and 3 mol of silver chloride were added, and the mixture was shaken in a lightless environment for 8 hours. After the reaction was completed, the solution after the reaction was filtered, decompressed and dried to obtain tri-n-nonylethylammonium chloride, which has the following structural formula:

[0058]

[0059] The present invention is further described below in conjunction with the examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. In this example, unless otherwise specified, the chemical reagents and raw materials used are all commercially available; the discarded shells involved in the examples are tetrahedral clam shells from the beach.

[0060] Comparative Example 1

[0061] The following steps are involved:

[0062] The discarded shells are calcined and crushed to obtain shell powder, and 20% dilute hydrochloric acid is added to the shell powder to dissolve it until no bubbles are generated, thereby obtaining a calcium chloride solution;

[0063] Heat the calcium chloride solution at 70°C to evaporate water until a small amount of white solid precipitates, and stop heating to obtain a saturated calcium chloride solution;

[0064] Commercially available citric acid monohydrate is added to water to prepare a citric acid solution with a citric acid mass content of 30%, and the citric acid solution is added dropwise to the saturated calcium chloride solution obtained in step 2 to obtain a precipitate. The precipitate is added dropwise until no more precipitation occurs, and the addition is stopped. After filtering and washing, a calcium citrate precipitate is obtained; the calcium citrate precipitate is dried and weighed, the molar amount of the calcium citrate is calculated, and a 10% sodium carbonate solution is prepared according to a ratio of 1:3.

[0065] The calcium citrate precipitate was added to the prepared sodium carbonate solution, and the mixture was reacted at 55° C. for 75 minutes to obtain a crude sodium citrate solution; the crude sodium citrate solution was then filtered, concentrated, washed and dried to obtain the sodium citrate product of Comparative Example 1.

[0066] Comparative Example 2

[0067] The following steps are involved:

[0068] The discarded shells are calcined and crushed to obtain shell powder, and 20% dilute hydrochloric acid is added to the shell powder to dissolve it until no bubbles are generated, thereby obtaining a calcium chloride solution;

[0069] Heat the calcium chloride solution at 70°C to evaporate water until a small amount of white solid precipitates, and stop heating to obtain a saturated calcium chloride solution;

[0070] Commercially available citric acid monohydrate is added to water to prepare a citric acid solution with a citric acid mass content of 30%, and the citric acid solution is added dropwise to the saturated calcium chloride solution obtained in step 2 to obtain a precipitate. The precipitate is added dropwise until no more precipitation occurs, and the addition is stopped. After filtering and washing, a calcium citrate precipitate is obtained; the calcium citrate precipitate is dried and weighed, the molar amount of the calcium citrate is calculated, and a 10% sodium carbonate solution is prepared according to a ratio of 1:3.

[0071] The calcium citrate precipitate was added to the prepared sodium carbonate solution, and the mixture was reacted at 85° C. for 75 min to obtain a crude sodium citrate solution; the crude sodium citrate solution was then filtered, concentrated, washed and dried to obtain the sodium citrate product of Comparative Example 2.

[0072] Comparative Example 3

[0073] The steps of Comparative Example 3 are substantially the same as those of Comparative Example 1, except that the double decomposition reaction time of Comparative Example 3 is 150 min.

[0074] Comparative Example 4

[0075] The steps of Comparative Example 4 are substantially the same as those of Comparative Example 2, except that the double decomposition reaction time of Comparative Example 4 is 150 min.

[0076] Comparative Example 5

[0077] The steps of Comparative Example 5 are substantially the same as those of Comparative Example 1, except that the metathesis reaction temperature of Comparative Example 5 is set to 40°C.

[0078] Comparative Example 6

[0079] The steps of Comparative Example 6 are substantially the same as those of Comparative Example 1, except that the metathesis reaction temperature of Comparative Example 6 is set to 30°C.

[0080] Embodiment 1

[0081] The following steps are involved:

[0082] A. Preparation of citric acid fermentation broth:

[0083] 35 g of citrus peel residue was weighed and crushed to obtain citrus peel residue powder; brewer's yeast was activated in a glucose solution to obtain activated yeast.

[0084] The activation process of brewer's yeast includes the following steps: weigh 1g of yeast, add 20mL of 10% glucose solution, activate at 38°C for 1h, and shake every 20min to obtain activated yeast.

[0085] Add citrus peel residue powder into water, then add activated yeast (at a ratio of 8% of the fermentation liquid mass), mix well, cover the bottle mouth with 3 layers of gauze, and then put it into a 30°C incubator for fermentation. During the fermentation process, measure the sugar content of the fermentation liquid every day until the residual sugar content no longer changes, and then end the fermentation.

[0086] The fermentation liquid after fermentation is boiled for 5 minutes, and then the heating is stopped. When the temperature of the fermentation liquid drops below 70°C, the liquid is filtered, decolorized and centrifuged, and the supernatant obtained after centrifugation is obtained to obtain the citric acid fermentation liquid.

[0087] B. Preparation of tri-n-nonylethylammonium chloride catalyst:

[0088] Add 1 mol of tri-n-nonylamine to 100 mL of acetonitrile and continue stirring (150-500 r / min), then add 1 mol of ethyl bromide and react in a 25°C water bath for 6 hours. After the reaction is completed, mix the reacted solution with 200 mL of water, stir at 40°C for 8 minutes, and collect the oil phase after standing;

[0089] The collected oil phase was washed with 300 mL of 0.15 mol / L sodium hydroxide solution, 300 mL of 1.2 mol / L sodium chloride solution and 300 mL of deionized water, respectively, and then decompressed and dried to obtain tri-n-nonylethylammonium bromide;

[0090] The obtained tri-n-nonylethylammonium bromide was added to 200 mL of propanol, and then 1 g of A5 molecular sieve and 3 mol of silver chloride were added, and the mixture was shaken in a lightless environment for 8 h. After the reaction was completed, the solution after the reaction was filtered, decompressed and dried to obtain tri-n-nonylethylammonium chloride.

[0091] C. Preparation of sodium citrate:

[0092] The discarded shells are calcined and crushed to obtain shell powder, and 20% dilute hydrochloric acid is added to the shell powder to dissolve it until no bubbles are generated, thereby obtaining a calcium chloride solution;

[0093] Heat the calcium chloride solution at 70°C to evaporate water until a small amount of white solid precipitates, and stop heating to obtain a saturated calcium chloride solution;

[0094] The citric acid fermentation liquid prepared above is added dropwise to the saturated calcium chloride solution obtained in step 2 to obtain a precipitate, and the addition is stopped until the precipitate no longer precipitates. After filtering and washing, a calcium citrate precipitate is obtained; the calcium citrate precipitate is dried and weighed, the molar amount of calcium citrate is calculated, and a 10% sodium carbonate solution is prepared according to a ratio of 1:3.

[0095] The calcium citrate precipitate was added to the prepared sodium carbonate solution, and the tri-n-nonylethylammonium chloride catalyst prepared above was added (the mass of the added catalyst was 0.12% of the mass of the solution), and the mixture was reacted at 30° C. for 75 minutes to obtain a crude sodium citrate solution; the crude sodium citrate solution was then filtered, extracted, concentrated, washed and dried to obtain the sodium citrate product of Example 1.

[0096] Embodiment 2

[0097] The steps of Example 2 are substantially the same as those of Example 1, except that the metathesis reaction temperature of Example 2 is set to 20°C.

[0098] Embodiment 3

[0099] The steps of Example 3 are substantially the same as those of Example 1, except that the metathesis reaction temperature of Example 3 is set to 12°C.

[0100] Embodiment 4

[0101] The steps of Example 4 are substantially the same as those of Example 1, except that the metathesis reaction temperature of Example 3 is set to 55°C.

[0102] Embodiment 5

[0103] The following steps are involved:

[0104] A. Preparation of citric acid fermentation broth:

[0105] 35 g of citrus peel residue was weighed and crushed to obtain citrus peel residue powder; brewer's yeast was activated in a glucose solution to obtain activated yeast.

[0106] The activation process of brewer's yeast includes the following steps: weigh 1g of yeast, add 20mL of 10% glucose solution, activate at 38°C for 1h, and shake every 20min to obtain activated yeast.

[0107] Add citrus peel residue powder into water, then add activated yeast (at a ratio of 8% of the fermentation liquid mass), mix well, cover the bottle mouth with 3 layers of gauze, and then put it into a 30°C incubator for fermentation. During the fermentation process, measure the sugar content of the fermentation liquid every day until the residual sugar content no longer changes, and then end the fermentation.

[0108] The fermentation liquid after fermentation is boiled for 5 minutes, and then the heating is stopped. When the temperature of the fermentation liquid drops below 70°C, the liquid is filtered, decolorized and centrifuged, and the supernatant obtained after centrifugation is obtained to obtain the citric acid fermentation liquid.

[0109] B. Preparation of sodium citrate:

[0110] The discarded shells are calcined and crushed to obtain shell powder, and 20% dilute hydrochloric acid is added to the shell powder to dissolve it until no bubbles are generated, thereby obtaining a calcium chloride solution;

[0111] Heat the calcium chloride solution at 70°C to evaporate water until a small amount of white solid precipitates, and stop heating to obtain a saturated calcium chloride solution;

[0112] The citric acid fermentation liquid prepared above is added dropwise to the saturated calcium chloride solution obtained in step 2 to obtain a precipitate, and the addition is stopped until the precipitate no longer precipitates. After filtering and washing, a calcium citrate precipitate is obtained; the calcium citrate precipitate is dried and weighed, the molar amount of calcium citrate is calculated, and a 10% sodium carbonate solution is prepared according to a ratio of 1:3.

[0113] The calcium citrate precipitate is added to the prepared sodium carbonate solution, and dodecyltrimethylammonium chloride catalyst is added at the same time (the mass of the added catalyst is 0.12% of the mass of the solution), and the reaction is carried out at 30° C. for 75 minutes to obtain a crude sodium citrate solution; the crude sodium citrate solution is then filtered, extracted, concentrated, washed and dried to obtain the sodium citrate product of Example 5.

[0114] Embodiment 6

[0115] The steps of Example 6 are substantially the same as those of Example 5, except that Example 6 uses n-decyltrimethylammonium chloride instead of dodecyltrimethylammonium chloride as a catalyst.

[0116] Embodiment 7

[0117] The steps of Example 7 are substantially the same as those of Example 5, except that Example 7 uses tri-n-nonylmethylammonium chloride instead of dodecyltrimethylammonium chloride as a catalyst.

[0118] Embodiment 8

[0119] The following steps are involved:

[0120] A. Preparation of tri-n-nonylethylammonium chloride catalyst:

[0121] Add 1 mol of tri-n-nonylamine to 100 mL of acetonitrile and continue stirring (150-500 r / min), then add 1 mol of ethyl bromide and react in a 25°C water bath for 6 hours. After the reaction is completed, mix the reacted solution with 200 mL of water, stir at 40°C for 8 minutes, and collect the oil phase after standing;

[0122] The collected oil phase was washed with 300 mL of 0.15 mol / L sodium hydroxide solution, 300 mL of 1.2 mol / L sodium chloride solution and 300 mL of deionized water, respectively, and then decompressed and dried to obtain tri-n-nonylethylammonium bromide;

[0123] The obtained tri-n-nonylethylammonium bromide was added to 200 mL of propanol, and then 1 g of A5 molecular sieve and 3 mol of silver chloride were added, and the mixture was shaken in a lightless environment for 8 h. After the reaction was completed, the solution after the reaction was filtered, decompressed and dried to obtain tri-n-nonylethylammonium chloride.

[0124] B. Preparation of sodium citrate:

[0125] The discarded shells are calcined and crushed to obtain shell powder, and 20% dilute hydrochloric acid is added to the shell powder to dissolve it until no bubbles are generated, thereby obtaining a calcium chloride solution;

[0126] Heat the calcium chloride solution at 70°C to evaporate water until a small amount of white solid precipitates, and stop heating to obtain a saturated calcium chloride solution;

[0127] Commercially available citric acid monohydrate is added to water to prepare a citric acid solution with a citric acid mass content of 30%, and the citric acid solution is added dropwise to the saturated calcium chloride solution obtained in step 2 to obtain a precipitate. The precipitate is added dropwise until no more precipitation occurs, and the addition is stopped. After filtering and washing, a calcium citrate precipitate is obtained; the calcium citrate precipitate is dried and weighed, the molar amount of the calcium citrate is calculated, and a 10% sodium carbonate solution is prepared according to a ratio of 1:3.

[0128] The calcium citrate precipitate is added to the prepared sodium carbonate solution, and the tri-n-nonylethylammonium chloride catalyst prepared above is added (the mass of the added catalyst is 0.12% of the mass of the solution), and the mixture is reacted at 30° C. for 75 minutes to obtain a crude sodium citrate solution; the crude sodium citrate solution is then filtered, extracted, concentrated, washed and dried to obtain the sodium citrate product of Example 8.

[0129] The yields of Examples 1 to 8 and Comparative Examples 1 to 6 and the content of strontium in the calcium citrate intermediate product and the sodium citrate final product were determined.

[0130] The yields of Examples 1 to 8 and Comparative Examples 1 to 6 are determined by calculating the difference in sodium carbonate concentration; at the same time, the calcium citrate intermediate products and sodium citrate final products of Examples 1 to 8 and Comparative Examples 1 to 6 are sampled, and the content of strontium is quantitatively analyzed by inductively coupled plasma emission spectrometry; specifically, 0.25 g of each of the calcium citrate intermediate products and sodium citrate final products of Examples 1 to 8 and Comparative Examples 1 to 6 are weighed and placed in a pressure vessel, 2 mL of nitric acid and 1 mL of water are added, and then sealed and placed in an oven at 150 ° C., constant temperature for 2 h, taken out for cooling and then moved into a volumetric flask, and determined after constant volume. The content of strontium is calculated according to the concentration of the standard and the sample dilution multiple.

[0131] The yields of Examples 1 to 8 and Comparative Examples 1 to 6 and the contents of strontium in the calcium citrate intermediate product and the sodium citrate final product are shown in the following table:

[0132]

[0133] According to the results of Comparative Examples 1 to 8 and Comparative Examples 1 to 5, it can be seen that the alkylammonium chloride phase transfer catalyst provided by the present invention can reduce the reaction temperature of the metathesis reaction to a large extent; when no catalyst is added, the metathesis reaction is at an optimal reaction temperature of about 55°C, and the addition of the alkylammonium chloride catalyst allows it to have a higher yield at room temperature, thereby not only reducing energy consumption, but also reducing the reaction device in the expanded production, thereby reducing costs. In addition, according to the results of Comparative Examples 1 to 4 and Examples 5 to 7, it can be seen that the present invention prepares a catalyst with good catalytic ability for the reaction of strontium citrate to generate sodium citrate, and under the action of the catalyst, more than 95% of strontium in the sodium citrate product can be removed, with significant effects.

[0134] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing the disclosure disclosed herein. The present invention is intended to cover any variations, uses or adaptations of the present invention that follow the general principles of the present invention and include common knowledge or customary technical means in the art that are not disclosed by the present invention.

Claims

1. A method for purifying and preparing sodium citrate, characterized in that: The following steps are involved: Step 1: calcining and crushing the discarded shells to obtain shell powder; Step 2: adding dilute hydrochloric acid to the shell powder to obtain a calcium chloride solution; heating the calcium chloride solution at 60° C. to 80° C. to evaporate water to obtain a saturated calcium chloride solution; Step 3: adding citric acid and / or citric acid fermentation broth to the saturated calcium chloride solution to obtain calcium citrate precipitate; Step 4: adding the calcium citrate precipitate to the sodium carbonate solution, and adding a phase transfer catalyst at the same time, reacting for 40-120 minutes, and obtaining a crude sodium citrate solution through double decomposition reaction; the phase transfer catalyst is tri-n-nonylethylammonium chloride; Step 5: filtering, extracting, concentrating, washing and drying the crude sodium citrate solution to obtain a sodium citrate product.

2. The method for purifying and preparing sodium citrate according to claim 1, characterized in that: The discarded shells are oyster shells, blue mussel shells, clam shells, quadrangular clam shells or pear-shaped ring-shaped snails.

3. The method for purifying and preparing sodium citrate according to claim 1, characterized in that: The citric acid fermentation broth is prepared by the following steps: Step 3a-1: crushing citrus peel residue to obtain citrus peel residue powder; activating brewer's yeast in a glucose solution to obtain activated yeast; Step 3a-2: adding the citrus peel residue powder to water, and then adding the activated yeast, and fermenting and culturing at 25° C.-35° C.; during the fermentation process, measuring the sugar content of the fermentation liquid every day until the residual sugar content no longer changes, and then ending the fermentation; Step 3a-3: Boil the fermentation liquid after fermentation for 3-8 minutes, stop heating, reduce the temperature of the fermentation liquid to below 70°C, filter, decolorize and centrifuge, and obtain the supernatant after centrifugation to obtain the citric acid fermentation liquid.

4. The method for purifying and preparing sodium citrate according to claim 1, characterized in that: In step 4, the concentration of the sodium carbonate solution is 8%-16%.

5. The method for purifying and preparing sodium citrate according to claim 1, characterized in that: In step 4, the reaction temperature is 5°C-80°C.

6. The method for purifying and preparing sodium citrate according to claim 4, characterized in that: In step 4, the reaction temperature is 20°C-30°C.

7. The method for purifying and preparing sodium citrate according to claim 1, characterized in that: The tri-n-nonylethylammonium chloride is prepared by the following steps: Step 4a-1: Add tri-n-nonylamine to acetonitrile and continue stirring, then add ethyl bromide, react at 20°C-40°C for 4-12 hours, after the reaction is completed, mix the reacted solution with water, stir at 30°C-45°C for 5-20 minutes, and collect the oil phase after standing; Step 4a-2: washing the collected oil phase with a sodium hydroxide solution, a sodium chloride solution and water respectively, and then reducing the pressure and draining to obtain tri-n-nonylethylammonium bromide; Step 4a-3: Add the obtained tri-n-nonylethylammonium bromide to an organic solvent, then add A5 molecular sieve and silver chloride, and shake in a lightless environment for 4-12 hours. After the reaction is completed, filter, reduce the pressure and drain the solution to obtain tri-n-nonylethylammonium chloride.

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