Preparation method of high-purity needle-like sodium sesquicarbonate crystal

Abstract

The application discloses a preparation method of high-purity needle-shaped sodium sesquicarbonate crystals and relates to the technical field of inorganic salt crystallization treatment. The sodium sesquicarbonate crystal mother liquor is prepared by dissolving sodium carbonate and sodium bicarbonate, a complex additive composed of polyvinylpyrrolidone and sodium dodecyl sulfate is added into the crystal mother liquor, and crystallization is carried out under the synergistic action of pulse ultrasonic waves, so that needle-shaped sodium sesquicarbonate crystal slurry with a high length-diameter ratio is obtained. After solid-liquid separation, the obtained needle-shaped sodium sesquicarbonate crystal filter cake is washed by anhydrous ethanol under low-temperature conditions, residual moisture among the crystals and on the crystal surface is replaced, the crystals washed by anhydrous ethanol are placed in a supercritical carbon dioxide environment for static drying, the whole-chain anti-agglomeration protection of the needle-shaped sodium sesquicarbonate crystals from crystallization growth to the final dried products is realized, thermal decomposition of the sodium sesquicarbonate is avoided, and the method is suitable for high-quality industrialized preparation of thermosensitive hydrated salts.

Description

Technical Field

[0001] This invention relates to the field of inorganic salt crystallization technology, specifically a method for preparing high-purity needle-shaped sodium sesquicarbonate crystals. Background Technology

[0002] Sodium sesquicarbonate is an important inorganic fine chemical product. Due to its excellent pH buffering properties, calcium and magnesium ion chelating ability, and moderate alkalinity, it has wide applications in high-end detergent auxiliaries, specialty glass clarifying agents, precision electronic cleaning agents, and food additives. Compared with ordinary granular or irregularly crystalline sodium sesquicarbonate, needle-shaped sodium sesquicarbonate crystals have a larger specific surface area, faster dissolution rate, and better flowability and bulk density, which can significantly improve the formulation uniformity and application efficiency of downstream products.

[0003] Currently, the preparation of needle-shaped sodium sesquicarbonate crystals mainly faces the following challenges: The problem of precise control of crystal morphology is that needle-shaped crystals need to grow in a specific direction along a crystal axis during the crystallization process to obtain a high aspect ratio. However, conventional cooling crystallization or evaporation crystallization methods are difficult to effectively control the relative growth rate of each crystal face, often resulting in products that are short columnar, plate-like or dendritic, with low aspect ratio and uneven morphology.

[0004] The agglomeration problem of crystalline products: due to their larger aspect ratio and specific surface area, needle-shaped crystals have a significantly higher surface energy than equiaxed crystals. In the later stages of crystallization and in subsequent unit operations such as filtration and drying, they are prone to secondary agglomeration and co-crystallization, thus losing the morphological advantages and dispersion performance of needle-shaped single crystals.

[0005] To address the aforementioned issues, existing technologies regulate crystal growth by adding a single surfactant. However, the regulatory effect of a single surfactant is limited, making it difficult to stably obtain needle-like crystals with an aspect ratio greater than 20. In terms of preventing agglomeration, existing methods mainly include freeze-drying, vacuum drying, or adding anti-caking agents. Although freeze-drying can better maintain crystal morphology, the equipment investment and operating costs are high, making it difficult to use for the industrial production of bulk inorganic salts. Adding anti-caking agents introduces exogenous substances, affecting the purity of the product.

[0006] Therefore, it is of great significance to develop a method for the stable preparation of needle-shaped sodium sesquicarbonate crystals with high aspect ratio, high dispersibility, and high purity. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing high-purity needle-shaped sodium sesquicarbonate crystals. The method involves dissolving sodium carbonate and sodium bicarbonate to prepare a crystallization mother liquor, adding a compound additive composed of polyvinylpyrrolidone and sodium dodecyl sulfate to the mother liquor, and crystallizing under the synergistic effect of pulsed ultrasound to obtain a high aspect ratio needle-shaped sodium sesquicarbonate slurry. After solid-liquid separation, the obtained needle-shaped sodium sesquicarbonate crystal filter cake is subjected to a three-stage displacement washing with anhydrous ethanol at low temperature to replace residual moisture between crystals and on the crystal surface. The crystals washed with anhydrous ethanol are then placed in a supercritical carbon dioxide environment for static drying. This method achieves full-chain anti-agglomeration protection for needle-shaped sodium sesquicarbonate crystals from crystallization growth to final dried product, avoiding the thermal decomposition of sodium sesquicarbonate. It is suitable for the high-quality industrial preparation of heat-sensitive hydrated salts.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing high-purity needle-shaped sodium sesquicarbonate crystals, the specific steps of which are as follows: S100. Dissolve sodium carbonate and sodium bicarbonate in deionized water at a molar ratio of 1:1 and stir until completely dissolved to prepare a mixed solution with a total alkali concentration of 1.2-1.8 mol / L. Filter to remove insoluble impurities from the mixed solution to obtain a clear crystallization mother liquor. S200. Add PVP-SDS compound additive to the crystallization mother liquor, stir until the compound additive is completely dissolved and uniformly dispersed in the crystallization mother liquor, heat the crystallization mother liquor to 35-45℃, apply pulsed ultrasound to the crystallization mother liquor for synergistic crystallization, and the crystallization time is 4-6h. S300. After the crystallization process of S200 is completed, the resulting crystallization slurry is vacuum filtered to achieve solid-liquid separation and obtain needle-shaped sodium sesquicarbonate crystal filter cake. S400. The needle-shaped sodium sesquicarbonate crystal filter cake is washed with anhydrous ethanol at 5°C in a three-stage displacement process to replace the water adsorbed between the needle-shaped sodium sesquicarbonate crystal particles and on the crystal surface. S500: The needle-shaped sodium sesquicarbonate crystal filter cake, after being washed by S400, is placed in a supercritical drying kettle for static drying to remove residual anhydrous ethanol from the needle-shaped sodium sesquicarbonate crystal filter cake. After depressurization, high-purity needle-shaped sodium sesquicarbonate crystal product is obtained.

[0009] Furthermore, in S100, the operation of filtering out insoluble impurities in the mixed solution is as follows: the mixed solution is pressure filtered using a microporous filter membrane with a pore size of 0.45 μm to remove insoluble solid particles with a particle size > 0.45 μm from the mixed solution.

[0010] Furthermore, in S200, the amount of PVP-SDS compound additive added is 0.05%-0.15% of the total mass of the crystallization mother liquor.

[0011] Furthermore, in S200, the PVP-SDS compound additive is composed of polyvinylpyrrolidone PVP-K30 and sodium dodecyl sulfate SDS in a mass ratio of 1:0.3.

[0012] Furthermore, in S200, the frequency of the pulsed ultrasound is 20-40kHz, and the pulse mode is ultrasound operation for 1-2 seconds followed by an ultrasound interval of 1-2 seconds.

[0013] Furthermore, in S400, the three-stage replacement washing operation is as follows: First stage washing: Add the first part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake, stir and wash at 5℃ for 10-15 min, and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the first stage washing. Second stage washing: Add the second part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the first stage washing, stir and wash at 5℃ for 10-15 minutes, and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the second stage washing. Third-stage washing: Add the third part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing, stir and wash at 5°C for 10-15 minutes, and then filter to obtain the washed needle-shaped sodium sesquicarbonate crystal filter cake.

[0014] Furthermore, in S400, the amount of anhydrous ethanol used in each washing stage is 1.5-2.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed.

[0015] Furthermore, the specific parameters of S500 are as follows: CO2 is introduced until the pressure inside the supercritical drying vessel reaches 7.5-8.5 MPa, the temperature inside the supercritical drying vessel is raised to 31-35℃, the needle-shaped sodium sesquicarbonate crystal filter cake is statically dried for 30-60 minutes, and after drying, high-purity needle-shaped sodium sesquicarbonate crystal product is obtained.

[0016] Furthermore, the compounding process of the PVP-SDS compound additive is as follows: Weigh out polyvinylpyrrolidone powder and sodium dodecyl sulfate powder at a mass ratio of 1:0.3. Add the weighed polyvinylpyrrolidone powder to deionized water at a temperature of 25-30℃, stir until completely dissolved, and prepare a polyvinylpyrrolidone aqueous solution with a mass concentration of 5%-10%. Add the weighed sodium dodecyl sulfate powder to the polyvinylpyrrolidone aqueous solution and stir at 25-30°C for 30-45 minutes. The PVP-SDS compound additive is obtained when sodium dodecyl sulfate is completely dissolved and a homogeneous and transparent mixed solution is formed.

[0017] Compared with existing technologies, this method for preparing high-purity needle-shaped sodium sesquicarbonate crystals has the following advantages: This invention achieves precise and stable control of the aspect ratio of sodium sesquicarbonate crystals through the synergistic effect of PVP-SDS compound additives and pulsed ultrasound. Polyvinylpyrrolidone (PVP-K30) molecules preferentially adsorb onto the side crystal faces, effectively inhibiting radial growth of the crystals. Sodium dodecyl sulfate (SDS) reduces the solid-liquid interfacial tension, improves the wettability of the crystal surface, and promotes uniform adsorption of PVP. The cavitation effect generated by pulsed ultrasound accelerates the transport of solute molecules along the crystal axis, promoting axial growth. The synergistic effect of these three factors results in uniform and consistent morphology of the prepared needle-shaped sodium sesquicarbonate crystals.

[0018] This invention employs an ethanol-supercritical CO2 combined solvent replacement anti-agglomeration drying method, which solves the problems of breakage and agglomeration during the drying process of needle-shaped crystals. Through three-stage anhydrous ethanol replacement washing under low-temperature conditions, the water adsorbed between crystal particles and on the surface is replaced, avoiding the capillary contraction force generated by the high surface tension of water during the drying process. The low surface tension characteristics of supercritical CO2 are used for static drying, eliminating the influence of capillary contraction force, so that the needle-shaped crystals maintain an intact one-dimensional structure during the drying process, and the discharged crystals are loose and do not agglomerate.

[0019] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from an examination of the following, or may be learned from the practice of the invention. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0021] Figure 1 A flowchart illustrating the steps involved in preparing high-purity needle-shaped sodium sesquicarbonate crystals; Figure 2 This is a diagram illustrating the compounding process of the PVP-SDS compound additive in S200 of this embodiment of the invention. Detailed Implementation

[0022] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structure, features and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0023] To address the challenges of stable aspect ratio control, easy agglomeration and breakage during crystal growth and drying, and insufficient product purity for high-end applications in existing needle-shaped sodium sesquicarbonate crystal preparation methods, this invention first describes its application scenarios. This invention is primarily applied in fields requiring high crystal morphology and purity, such as food additives, pharmaceutical intermediates, high-end detergent auxiliaries, industrial water treatment agents, composite material reinforcing agents, high-efficiency adsorbent materials, and catalyst supports. In these scenarios, traditional preparation methods using natural alkali extraction or single surfactant-controlled crystallization suffer from drawbacks: low purity and irregular crystal morphology in the natural alkali method; severe breakage and agglomeration of needle-shaped crystals due to capillary contraction during conventional drying; and the limited ability of single surfactants to inhibit radial crystal growth, failing to achieve a stable aspect ratio greater than 20. Conventional vacuum drying and spray drying techniques cannot fundamentally eliminate the influence of capillary contraction, thus hindering the industrialization of high-purity, high aspect ratio needle-shaped sodium sesquicarbonate crystals in high-end applications.

[0024] like Figure 1 As shown, this invention provides a method for preparing high-purity needle-shaped sodium sesquicarbonate crystals. The specific steps of this method are as follows: S100. Dissolve sodium carbonate and sodium bicarbonate in deionized water at a molar ratio of 1:1 and stir until completely dissolved to prepare a mixed solution with a total alkali concentration of 1.2-1.8 mol / L. Filter to remove insoluble impurities from the mixed solution to obtain a clear crystallization mother liquor. S200. Add PVP-SDS compound additive to the crystallization mother liquor, stir until the compound additive is completely dissolved and uniformly dispersed in the crystallization mother liquor, heat the crystallization mother liquor to 35-45℃, apply pulsed ultrasound to the crystallization mother liquor for synergistic crystallization, and the crystallization time is 4-6h. S300. After the crystallization process of S200 is completed, the resulting crystallization slurry is vacuum filtered to achieve solid-liquid separation and obtain needle-shaped sodium sesquicarbonate crystal filter cake. S400. The needle-shaped sodium sesquicarbonate crystal filter cake is washed with anhydrous ethanol at 5°C in a three-stage displacement process to replace the water adsorbed between the needle-shaped sodium sesquicarbonate crystal particles and on the crystal surface. S500: The needle-shaped sodium sesquicarbonate crystal filter cake, after being washed by S400, is placed in a supercritical drying kettle for static drying to remove residual anhydrous ethanol from the needle-shaped sodium sesquicarbonate crystal filter cake. After depressurization, high-purity needle-shaped sodium sesquicarbonate crystal product is obtained.

[0025] Among them, such as Figure 2 As shown, the compounding process of the PVP-SDS compound additive in S200 is as follows: Weigh out polyvinylpyrrolidone powder and sodium dodecyl sulfate powder at a mass ratio of 1:0.3. Add the weighed polyvinylpyrrolidone powder to deionized water at a temperature of 25-30℃, stir until completely dissolved, and prepare a polyvinylpyrrolidone aqueous solution with a mass concentration of 5%-10%. Add the weighed sodium dodecyl sulfate powder to the polyvinylpyrrolidone aqueous solution and stir at 25-30°C for 30-45 minutes. The PVP-SDS compound additive is obtained when sodium dodecyl sulfate is completely dissolved and a homogeneous and transparent mixed solution is formed.

[0026] Unified Test Instructions The sodium sesquicarbonate crystal products prepared in all embodiments and comparative examples of this invention were subjected to performance testing using the following uniform method: Crystal aspect ratio determination: The crystal morphology was observed using a scanning electron microscope. 100 complete crystal particles were randomly selected under a field of view with a magnification of 500x. The length and maximum diameter of each crystal were measured and the aspect ratio of each crystal was calculated. Finally, the arithmetic mean of the aspect ratios of the 100 crystals was taken as the aspect ratio of the sample.

[0027] Single crystal dispersion retention rate determination: Using the above-mentioned scanning electron microscope at a magnification of 200x, five different regions were randomly selected. The total number of crystal particles and the number of independently dispersed single crystal particles in each region were counted. The single crystal dispersion of each region was calculated. Finally, the arithmetic mean of the single crystal dispersion of the five regions was taken as the single crystal dispersion retention rate of the sample.

[0028] Product purity determination: The purity of sodium sesquicarbonate in the product was determined by titration with hydrochloric acid standard solution. 0.5g of dried sample was weighed and placed in a 250mL Erlenmeyer flask. 50mL of deionized water was added to dissolve the sample. 2-3 drops of methyl orange indicator were added. The solution was titrated with 0.1mol / L hydrochloric acid standard solution until the solution changed from yellow to orange, which was the endpoint. The total alkali content was calculated based on the amount of hydrochloric acid standard solution consumed, and the product purity was then calculated based on the chemical composition of sodium sesquicarbonate.

[0029] Crystal morphology observation: The overall morphology of the crystal was observed using the scanning electron microscope described above, and the shape, homogeneity, aggregation and fracture of the crystal were recorded.

[0030] Example 1 In this embodiment, a crystallization mother liquor with a total alkali concentration of 1.5 mol / L was prepared by mixing sodium carbonate and sodium bicarbonate in a molar ratio of 1:1. 0.1% (mass ratio of PVP-SDS to PVP-K30 to SDS 1:0.3) was added. The mixture was then subjected to pulsed ultrasonic treatment at 40°C with a frequency of 28 kHz and a 1-second interval between cycles for 5 hours to achieve stable crystallization. After three stages of anhydrous ethanol replacement washing at 5°C (each stage using twice the mass of the filter cake, washing for 12 minutes), the mixture was statically dried under supercritical CO2 at 8 MPa and 32°C for 40 minutes, with pressure released at a rate of 0.5 MPa / min. This process resulted in the stable preparation of needle-shaped sodium sesquicarbonate crystals with high aspect ratio, high dispersibility, and high purity.

[0031] S100. Dissolve sodium carbonate and sodium bicarbonate in deionized water at a molar ratio of 1:1 and stir until completely dissolved to prepare a mixed solution with a total alkali concentration of 1.5 mol / L. Filter the mixed solution under pressure using a microporous membrane with a pore size of 0.45 μm to remove insoluble solid particles with a particle size greater than 0.45 μm, and obtain a clear mother liquor for crystallization.

[0032] S200. Pre-preparation of PVP-SDS compound additive: Weigh polyvinylpyrrolidone (PVP-K30) powder and sodium dodecyl sulfate (SDS) powder at a mass ratio of 1:0.3. Add the weighed PVP-K30 powder to deionized water at 28°C and stir until completely dissolved to prepare an 8% PVP-SDS aqueous solution. Slowly add the weighed sodium dodecyl sulfate (SDS) powder to the PVP-SDS aqueous solution and stir continuously at 28°C for 38 minutes until the sodium dodecyl sulfate (SDS) is completely dissolved and a homogeneous and transparent mixed solution is formed, thus obtaining the PVP-SDS compound additive.

[0033] The above-mentioned PVP-SDS compound additive was added to the crystallization mother liquor obtained by S100, and the amount added was 0.1% of the total mass of the crystallization mother liquor. The mixture was stirred until the PVP-SDS compound additive was completely dissolved and uniformly dispersed in the crystallization mother liquor. The crystallization mother liquor was heated to 40°C, and pulsed ultrasound was applied to the crystallization mother liquor for synergistic crystallization. The crystallization time was 5 hours. The frequency of the pulsed ultrasound was 28 kHz, and the pulse mode was 1 second of ultrasound operation followed by 1 second of ultrasound interval.

[0034] After the crystallization process of S200 is completed, the resulting crystallization slurry is vacuum filtered at a vacuum degree of 0.08 MPa to achieve solid-liquid separation and obtain needle-shaped sodium sesquicarbonate crystal filter cake.

[0035] S400. The needle-shaped sodium sesquicarbonate crystal filter cake is subjected to a three-stage displacement washing with anhydrous ethanol at 5°C. First washing: Add the first part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake. The amount of anhydrous ethanol is twice the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 12 minutes and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the first washing. Second-stage washing: Add a second part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the first-stage washing. The amount of anhydrous ethanol is twice the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 12 minutes and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing. Third-stage washing: Add a third part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing. The amount of anhydrous ethanol is twice the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 12 minutes, then filter to obtain the washed needle-shaped sodium sesquicarbonate crystal filter cake.

[0036] S500: Place the needle-shaped sodium sesquicarbonate crystal filter cake, washed in S400, into a supercritical drying reactor. First, introduce liquid CO2 into the supercritical drying reactor to expel the air inside. Then, continue to introduce liquid CO2 and increase the pressure until the pressure inside the supercritical drying reactor stabilizes at 8 MPa. Raise the temperature inside the supercritical drying reactor to 32°C and statically dry the needle-shaped sodium sesquicarbonate crystal filter cake for 40 minutes. After drying, slowly release the pressure inside the supercritical drying reactor to atmospheric pressure at a rate of 0.5 MPa / min. Remove the dried needle-shaped sodium sesquicarbonate crystals and immediately seal and package them to obtain a high-purity needle-shaped sodium sesquicarbonate crystal product.

[0037] Performance test results Crystal aspect ratio: 24.2; single crystal dispersion retention rate: 97.6%; product purity: 99.7%; crystal morphology: uniform, slender needle-like, without agglomeration or breakage.

[0038] In this embodiment, the growth direction of sodium sesquicarbonate crystals was precisely controlled through the synergistic effect of PVP-SDS compound additives and pulsed ultrasound, resulting in uniform needle-shaped crystals with a high aspect ratio of 24.2. At the same time, the ethanol-supercritical CO2 combined solvent replacement anti-agglomeration drying technology was used to effectively protect the one-dimensional structure of the crystals, achieving a single crystal dispersion retention rate of 97.6% and a product purity of up to 99.7%. This embodiment verifies that the prepared product can meet the application requirements of high-end fields such as food, medicine, and composite materials.

[0039] Example 2 In this embodiment, a crystallization mother liquor with a total alkali concentration of 1.2 mol / L was prepared by mixing sodium carbonate and sodium bicarbonate in a molar ratio of 1:1. 0.05% PVP-SDS compound additive was added, and the mixture was subjected to pulsed ultrasonic treatment at 35°C with a frequency of 20 kHz and a 1-second interval between cycles for 6 hours to achieve synergistic crystallization. After washing with three stages of anhydrous ethanol at 5°C (each stage using 1.5 times the mass of the filter cake, washing for 10 minutes), the mixture was statically dried with supercritical CO2 at 7.5 MPa and 31°C for 60 minutes, with pressure released at a rate of 0.3 MPa / min. This successfully prepared high-purity needle-shaped sodium sesquicarbonate crystals that met the requirements.

[0040] S100. Dissolve sodium carbonate and sodium bicarbonate in deionized water at a molar ratio of 1:1 and stir until completely dissolved to prepare a mixed solution with a total alkali concentration of 1.2 mol / L. Use a microporous membrane with a pore size of 0.45 μm to filter the mixed solution under pressure to remove insoluble solid particles with a particle size greater than 0.45 μm, and obtain a clear crystallization mother liquor.

[0041] S200. Pre-preparation of PVP-SDS compound additive: Weigh polyvinylpyrrolidone (PVP-K30) powder and sodium dodecyl sulfate (SDS) powder at a mass ratio of 1:0.3. Add the weighed PVP-K30 powder to deionized water at 25°C and stir until completely dissolved to prepare a 5% PVP-SDS aqueous solution. Slowly add the weighed sodium dodecyl sulfate (SDS) powder to the PVP-SDS aqueous solution and stir continuously at 25°C for 30 minutes until the sodium dodecyl sulfate (SDS) is completely dissolved and a homogeneous and transparent mixed solution is formed, thus obtaining the PVP-SDS compound additive.

[0042] The above-mentioned PVP-SDS compound additive was added to the crystallization mother liquor obtained by S100, and the amount added was 0.05% of the total mass of the crystallization mother liquor. The mixture was stirred until the PVP-SDS compound additive was completely dissolved and uniformly dispersed in the crystallization mother liquor. The crystallization mother liquor was heated to 35°C, and pulsed ultrasound was applied to the crystallization mother liquor for synergistic crystallization. The crystallization time was 6 hours. The frequency of the pulsed ultrasound was 20 kHz, and the pulse mode was 1 second of ultrasound operation followed by 1 second of ultrasound interval.

[0043] After the crystallization process of S200 is completed, the resulting crystallization slurry is vacuum filtered at a vacuum degree of 0.08 MPa to achieve solid-liquid separation and obtain needle-shaped sodium sesquicarbonate crystal filter cake.

[0044] S400. The needle-shaped sodium sesquicarbonate crystal filter cake is subjected to a three-stage displacement washing with anhydrous ethanol at 5°C. First-stage washing: Add the first part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake. The amount of anhydrous ethanol is 1.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 10 minutes and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the first-stage washing. Second-stage washing: Add a second part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the first-stage washing. The amount of anhydrous ethanol is 1.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 10 minutes and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing. Third-stage washing: Add a third part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing. The amount of anhydrous ethanol is 1.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 10 minutes, then filter to obtain the washed needle-shaped sodium sesquicarbonate crystal filter cake.

[0045] S500: Place the needle-shaped sodium sesquicarbonate crystal filter cake, washed in S400, into a supercritical drying reactor. First, introduce liquid CO2 into the supercritical drying reactor to expel the air inside. Then, continue to introduce liquid CO2 and increase the pressure until the pressure inside the supercritical drying reactor stabilizes at 7.5 MPa. Raise the temperature inside the supercritical drying reactor to 31°C and statically dry the needle-shaped sodium sesquicarbonate crystal filter cake for 60 minutes. After drying, slowly release the pressure inside the supercritical drying reactor to atmospheric pressure at a rate of 0.3 MPa / min. Remove the dried needle-shaped sodium sesquicarbonate crystals and immediately seal and package them to obtain the high-purity needle-shaped sodium sesquicarbonate crystal product.

[0046] Performance test results Crystal aspect ratio: 21.5; single crystal dispersion retention rate: 96.3%; product purity: 99.5%; crystal morphology: uniform needle-like, without obvious agglomeration and fracture.

[0047] This embodiment, under conditions of lower total alkali concentration, lower amount of compound additives, lower crystallization temperature and longer crystallization time, can still effectively control the crystal aspect ratio to be greater than 20, the single crystal dispersion retention rate to be higher than 96%, and the product purity to reach 99.5%, verifying the good adaptability and stability of the present invention.

[0048] Example 3 In this embodiment, a crystallization mother liquor with a total alkali concentration of 1.8 mol / L was prepared by mixing sodium carbonate and sodium bicarbonate in a molar ratio of 1:1. 0.15% PVP-SDS compound additive was added, and the mixture was subjected to pulsed ultrasonic treatment at 40 kHz frequency with a 2-second working and 2-second intermittent interval at 45°C for 4 hours to induce crystallization. After three stages of anhydrous ethanol replacement washing at 5°C (each stage using 2.5 times the mass of the filter cake, washing for 15 minutes), the mixture was statically dried under supercritical CO2 at 8.5 MPa and 35°C for 30 minutes, with pressure released at a rate of 0.7 MPa / min, successfully producing high-quality, high-purity needle-shaped sodium sesquicarbonate crystals.

[0049] S100. Dissolve sodium carbonate and sodium bicarbonate in deionized water at a molar ratio of 1:1 and stir until completely dissolved to prepare a mixed solution with a total alkali concentration of 1.8 mol / L. Filter the mixed solution under pressure using a microporous membrane with a pore size of 0.45 μm to remove insoluble solid particles with a particle size greater than 0.45 μm, and obtain a clear mother liquor for crystallization.

[0050] S200. Pre-preparation of PVP-SDS compound additive: Weigh polyvinylpyrrolidone (PVP-K30) powder and sodium dodecyl sulfate (SDS) powder at a mass ratio of 1:0.3. Add the weighed PVP-K30 powder to deionized water at 30°C and stir until completely dissolved to prepare a 10% PVP-SDS aqueous solution. Slowly add the weighed sodium dodecyl sulfate (SDS) powder to the PVP-SDS aqueous solution and stir continuously at 30°C for 45 minutes until the sodium dodecyl sulfate (SDS) is completely dissolved and a homogeneous and transparent mixed solution is formed, thus obtaining the PVP-SDS compound additive.

[0051] The above-mentioned PVP-SDS compound additive was added to the crystallization mother liquor obtained by S100, and the amount added was 0.15% of the total mass of the crystallization mother liquor. The mixture was stirred until the PVP-SDS compound additive was completely dissolved and uniformly dispersed in the crystallization mother liquor. The crystallization mother liquor was heated to 45°C, and pulsed ultrasound was applied to the crystallization mother liquor for synergistic crystallization. The crystallization time was 4 hours. The frequency of the pulsed ultrasound was 40 kHz, and the pulse mode was 2 seconds of ultrasound operation followed by 2 seconds of ultrasound interval.

[0052] After the crystallization process of S200 is completed, the resulting crystallization slurry is vacuum filtered at a vacuum degree of 0.08 MPa to achieve solid-liquid separation and obtain needle-shaped sodium sesquicarbonate crystal filter cake.

[0053] S400. The needle-shaped sodium sesquicarbonate crystal filter cake is subjected to a three-stage displacement washing with anhydrous ethanol at 5°C. First-stage washing: Add the first part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake. The amount of anhydrous ethanol is 2.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 15 minutes and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the first-stage washing. Second-stage washing: Add a second part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the first-stage washing. The amount of anhydrous ethanol is 2.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 15 minutes and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing. Third-stage washing: Add a third part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing. The amount of anhydrous ethanol is 2.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed. Stir and wash at 5°C for 15 minutes, then filter to obtain the washed needle-shaped sodium sesquicarbonate crystal filter cake.

[0054] S500: Place the needle-shaped sodium sesquicarbonate crystal filter cake, washed in S400, into a supercritical drying reactor. First, introduce liquid CO2 into the supercritical drying reactor to expel the air inside. Then, continue to introduce liquid CO2 and increase the pressure until the pressure inside the supercritical drying reactor stabilizes at 8.5 MPa. Raise the temperature inside the supercritical drying reactor to 35°C and statically dry the needle-shaped sodium sesquicarbonate crystal filter cake for 30 minutes. After drying, slowly release the pressure inside the supercritical drying reactor to atmospheric pressure at a rate of 0.7 MPa / min. Remove the dried needle-shaped sodium sesquicarbonate crystals and immediately seal and package them to obtain the high-purity needle-shaped sodium sesquicarbonate crystal product.

[0055] Performance test results Crystal aspect ratio: 23.1; single crystal dispersion retention rate: 96.9%; product purity: 99.6%; crystal morphology: uniform needle-like, without agglomeration or fracture.

[0056] In this embodiment, even under conditions of high total alkali concentration, high amount of compound additives, high crystallization temperature, and short crystallization time, the technical solution of the present invention can still stably prepare needle-shaped sodium sesquicarbonate crystals with an aspect ratio greater than 20, a single crystal dispersion retention rate of more than 96%, and a purity of more than 99.5%, verifying the robustness and reliability of the process of the present invention.

[0057] Comparative Example 1 The difference between this comparative example and Example 1 is that no PVP-SDS compound additive is added. The remaining steps are exactly the same as in Example 1. This example is used to verify the key role of the PVP-SDS compound additive in controlling the aspect ratio of crystals and preventing agglomeration during crystal growth.

[0058] Except for the absence of PVP-SDS compound additive in S200, all other steps are exactly the same as in Example 1.

[0059] Performance test results Crystal aspect ratio: 7.3; single crystal dispersion retention rate: 81.5%; product purity: 99.1%; crystal morphology: short columnar, severely agglomerated, with uneven particle size distribution.

[0060] In this comparative example, without the addition of PVP-SDS compound additives, crystallization was controlled solely by pulsed ultrasound. The results showed that the crystal aspect ratio was only 7.3, and the crystals exhibited severe agglomeration, resulting in a decrease in product purity. This demonstrates that the PVP-SDS compound additives are the core of this invention for achieving precise control of the crystal aspect ratio and preventing agglomeration during crystal growth.

[0061] Comparative Example 2 The difference between this comparative example and Example 1 is that pulsed ultrasound is not applied; the remaining steps are exactly the same as in Example 1. This example is used to verify the important role of pulsed ultrasound in promoting axial growth of crystals and improving the aspect ratio of crystals.

[0062] Except for the absence of pulsed ultrasound in S200, the remaining steps are exactly the same as in Example 1.

[0063] Performance test results Crystal aspect ratio: 12.1; single crystal dispersion retention rate: 84.8%; product purity: 99.2%; crystal morphology: short needle-like, partially agglomerated.

[0064] In this comparative example, crystallization was regulated solely by a PVP-SDS composite additive without the application of pulsed ultrasound. The results showed that the crystal aspect ratio was only 12.1, and partial agglomeration occurred. This fully demonstrates that pulsed ultrasound can significantly accelerate the transport of solute molecules along the crystal axis, promote axial crystal growth, and is an indispensable technique for obtaining needle-like crystals with high aspect ratios.

[0065] Comparative Example 3 The difference between this comparative example and Example 1 is that the ethanol-supercritical CO2 combined solvent displacement anti-agglomeration drying method is replaced with conventional vacuum drying at 60°C for 12 hours. The remaining steps are exactly the same as in Example 1. This is used to verify the key role of the drying technology of the present invention in preventing breakage and agglomeration of needle-shaped crystals during the drying process.

[0066] Except for replacing S400 and S500 with vacuum drying of the needle-shaped sodium sesquicarbonate crystal filter cake obtained in S300 at 60°C for 12 hours, all other steps are exactly the same as in Example 1.

[0067] Performance test results Crystal aspect ratio: 17.9; single crystal dispersion retention rate: 67.2%; product purity: 99.3%; crystal morphology: needle-like, with a large number of fractures and agglomerations, and severe damage to the one-dimensional structure.

[0068] This comparative example uses a conventional vacuum drying method. Although needle-shaped crystals with an aspect ratio of 17.9 were obtained during the crystallization process, a large number of crystals fractured and agglomerated after drying, and the single crystal dispersion retention rate was only 67.2%. The one-dimensional structure of the crystals was severely damaged. This proves that the ethanol-supercritical CO2 combined solvent replacement anti-agglomeration drying method of the present invention can effectively eliminate capillary contraction force during the drying process and protect the complete one-dimensional structure of the needle-shaped crystals.

[0069] To visually compare the performance differences between the various embodiments of the present invention and the comparative examples, and to clearly demonstrate the effects of each core technical feature, the performance test results of all samples are summarized in Table 1 below: Table 1: Performance test results of sodium sesquicarbonate crystals under different process conditions As shown in Table 1, the aspect ratio of the needle-shaped sodium sesquicarbonate crystals prepared in the three embodiments of this invention is consistently greater than 20, the single crystal dispersion retention rate is higher than 96%, the product purity is greater than 99.5%, and the crystal morphology is uniform. Comparative analysis shows that the PVP-SDS compound additive is the core for controlling the aspect ratio of the crystal and preventing growth agglomeration. Without it, the aspect ratio of the crystal drops sharply to 7.3 and severe agglomeration occurs. Pulsed ultrasound and compound additive have a significant synergistic effect. Using the compound additive alone can only obtain short needle-shaped crystals with an aspect ratio of 12.1. The ethanol-supercritical CO2 combined drying technology is the key to protecting the one-dimensional structure of the crystal. Conventional vacuum drying will cause more than 60% of the crystals to break and agglomerate. This technology achieves full-chain anti-agglomeration protection for needle-shaped sodium sesquicarbonate crystals from crystal growth to the final dried product.

[0070] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing high-purity needle-shaped sodium sesquicarbonate crystals, characterized in that, The specific steps of this method are as follows: S100. Dissolve sodium carbonate and sodium bicarbonate in deionized water at a molar ratio of 1:1 and stir until completely dissolved to prepare a mixed solution with a total alkali concentration of 1.2-1.8 mol / L. Filter to remove insoluble impurities from the mixed solution to obtain a clear crystallization mother liquor. S200. Add PVP-SDS compound additive to the crystallization mother liquor, stir until the compound additive is completely dissolved and uniformly dispersed in the crystallization mother liquor, heat the crystallization mother liquor to 35-45℃, apply pulsed ultrasound to the crystallization mother liquor for synergistic crystallization, and the crystallization time is 4-6h. S300. After the crystallization process of S200 is completed, the resulting crystallization slurry is vacuum filtered to obtain a filter cake of needle-shaped sodium sesquicarbonate crystals. S400. The needle-shaped sodium sesquicarbonate crystal filter cake is washed with anhydrous ethanol at 5°C in a three-stage displacement washing process to replace the water adsorbed between the needle-shaped sodium sesquicarbonate crystal particles and on the crystal surface. S500: The needle-shaped sodium sesquicarbonate crystal filter cake, after being washed by S400, is placed in a supercritical drying kettle for static drying to remove residual anhydrous ethanol from the needle-shaped sodium sesquicarbonate crystal filter cake. After depressurization, high-purity needle-shaped sodium sesquicarbonate crystal product is obtained.

2. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 1, characterized in that, In step S100, the operation of filtering out insoluble impurities in the mixed solution is as follows: the mixed solution is pressure filtered using a microporous filter membrane with a pore size of 0.45 μm to remove insoluble solid particles with a particle size > 0.45 μm from the mixed solution.

3. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 1, characterized in that, In S200, the amount of PVP-SDS compound additive added is 0.05%-0.15% of the total mass of the crystallization mother liquor.

4. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 1, characterized in that, In S200, the PVP-SDS compound additive is composed of polyvinylpyrrolidone PVP-K30 and sodium dodecyl sulfate SDS in a mass ratio of 1:0.

3.

5. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 1, characterized in that, In S200, the frequency of the pulsed ultrasound is 20-40kHz, and the pulse mode is ultrasound operation for 1-2 seconds followed by an ultrasound interval of 1-2 seconds.

6. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 1, characterized in that, In S400, the three-stage replacement washing operation is as follows: First stage washing: Add the first part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake, stir and wash at 5℃ for 10-15 min, and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the first stage washing. Second stage washing: Add the second part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the first stage washing, stir and wash at 5℃ for 10-15 minutes, and then filter to obtain the needle-shaped sodium sesquicarbonate crystal filter cake after the second stage washing. Third-stage washing: Add the third part of anhydrous ethanol to the needle-shaped sodium sesquicarbonate crystal filter cake after the second-stage washing, stir and wash at 5°C for 10-15 minutes, and then filter to obtain the washed needle-shaped sodium sesquicarbonate crystal filter cake.

7. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 6, characterized in that, In S400, the amount of anhydrous ethanol used in each washing stage is 1.5-2.5 times the mass of the needle-shaped sodium sesquicarbonate crystal filter cake to be washed.

8. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 1, characterized in that, The specific parameters of the S500 are as follows: CO2 is introduced until the pressure inside the supercritical drying vessel reaches 7.5-8.5 MPa, the temperature inside the supercritical drying vessel is raised to 31-35℃, the filter cake of needle-shaped sodium sesquicarbonate crystals is statically dried for 30-60 minutes, and after drying, high-purity needle-shaped sodium sesquicarbonate crystals are obtained.

9. The method for preparing high-purity needle-shaped sodium sesquicarbonate crystals according to claim 4, characterized in that, The compounding process of the PVP-SDS compound additive is as follows: Weigh out polyvinylpyrrolidone powder and sodium dodecyl sulfate powder at a mass ratio of 1:0.

3. Add the weighed polyvinylpyrrolidone powder to deionized water at a temperature of 25-30℃, stir until completely dissolved, and prepare a polyvinylpyrrolidone aqueous solution with a mass concentration of 5%-10%. Add the weighed sodium dodecyl sulfate powder to the polyvinylpyrrolidone aqueous solution and stir at 25-30°C for 30-45 minutes. The PVP-SDS compound additive is obtained when sodium dodecyl sulfate is completely dissolved and a homogeneous and transparent mixed solution is formed.

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