Sodium borohydride preparation method based on concentration and crystallization treatment

Abstract

The invention relates to the technical field of sodium borohydride preparation, in particular to a sodium borohydride preparation method based on concentration and crystallization treatment. Comprising the following steps: adding crude sodium borohydride and deionized water into a high-pressure kettle according to a mass ratio, stirring, and filtering to obtain extract liquor; pumping the extract liquor into a reverse osmosis system, and carrying out membrane concentration; the concentrated solution is transferred to a crystallization kettle, polyethylene glycol-4000 is added, gradient cooling is executed for crystallization, and in the second stage of gradient cooling, sodium borohydride seed crystals are added, and meanwhile ultrasonic waves are applied for auxiliary crystallization; pumping the crystallized slurry into a horizontal centrifugal machine, separating crystals, putting the obtained crystals into a double-cone rotary drying machine, and drying to obtain concentrated sodium borohydride crystals; a traditional evaporation process is replaced by reverse osmosis membrane low-temperature concentration, by-product sodium metaborate impurity pollution is eradicated, the source of the phenomenon that metal ions induce dendritic crystal growth is eliminated, high-purity sodium borohydride crystals can be obtained through single crystallization, and the product yield is increased.

Description

Technical Field

[0001] The invention relates to the technical field of sodium borohydride preparation, in particular to a method for preparing sodium borohydride based on concentrated crystallization treatment. Background Art

[0002] Sodium borohydride (NaBH4) is a widely used complex metal hydride reducing agent with extensive applications in organic and inorganic chemistry. Although its reducing performance for organic functional groups is weaker than that of lithium aluminum hydride, it offers many unique advantages. For example, in the presence of functional groups such as esters and amines, it can selectively reduce aldehydes, ketones, and acyl chlorides to alcohols, while ester and amine groups are reduced by lithium aluminum hydride. It can also be used in alkaline aqueous solutions and is safe to operate—advantages not matched by metal aluminum hydrides. Economically, sodium borohydride is less expensive than lithium aluminum hydride for equivalent reducing equivalents, enhancing its competitiveness as a commercial complex metal hydride reducing agent.

[0003] Traditional sodium borohydride preparation mainly adopts the Schlesinger method and its improved process, which usually uses metallic sodium, hydrogen and trimethyl borate as raw materials. The metallic sodium is first melted and dispersed in mineral oil to form sodium sand. It is then reacted with hydrogen at 300-400℃ and 10-20MPa high pressure to produce sodium hydride. The sodium hydride is then reacted with trimethyl borate in an inert solvent such as ether or tetrahydrofuran at 250-300℃ to produce sodium borohydride. The crude product is obtained through evaporation and concentration, cooling crystallization, filtration separation and other steps, and finally purified by multiple recrystallizations.

[0004] In the preparation process of sodium borohydride, concentration and crystallization are key purification steps. Conventional concentration methods in the prior art primarily rely on evaporation. However, high temperatures can easily lead to decomposition of sodium borohydride and increase the generation of sodium metaborate impurities, contaminating the crystallization system. Furthermore, the crystallization process often employs single-stage cooling or simple gradient cooling, resulting in sodium borohydride crystals with a wide particle size distribution and prone to agglomeration. Furthermore, residual sodium metaborate impurities require multiple recrystallizations to achieve a high purity, resulting in yield losses. Summary of the Invention

[0005] The present invention provides a method for preparing sodium borohydride based on concentrated crystallization treatment. The method replaces the traditional evaporation process with a reverse osmosis system gradient cooling and concentration process to eliminate sodium metaborate impurity contamination. The method is combined with a polypiperazineamide composite membrane for series circulation desalination to eliminate the root cause of metal ion-induced dendrite growth. High-purity sodium borohydride crystals can be obtained in a single crystallization, thereby solving the problems raised in the above-mentioned background technology, namely: In the existing sodium borohydride purification process, evaporation and concentration lead to a high decomposition rate of sodium borohydride, and the residual sodium metaborate impurity in the concentrate induces the growth of crystal dendrites, forcing multiple recrystallizations and causing yield loss.

[0006] To achieve the above object, the method for preparing sodium borohydride comprises the following steps: S1. Add crude sodium borohydride and deionized water in a mass ratio of 1:3-1:5 into an autoclave, stir at a constant temperature for 10-30 minutes, and filter to obtain an extract containing sodium borohydride; S2. Pumping the extract into a reverse osmosis system using a polypiperazineamide composite membrane to concentrate the extract to a sodium borohydride mass concentration of ≥20% under an operating pressure of 3-4 MPa and a membrane surface flow rate of 2-4 m / s; S3. The concentrated solution is transferred to a crystallization kettle, 0.005-0.02 wt% polyethylene glycol-4000 is added, and a gradient cooling process is performed in three stages: In the first stage, the crystallization kettle is kept at a constant temperature of 40°C for 1-2 hours. In the second stage, after constant temperature, the temperature is lowered to 25-28℃ at a rate of 4-6℃ / h. When the temperature reaches 26±0.5℃, sodium borohydride seeds with a particle size of 50-100 μm are added. The amount of sodium borohydride seeds added is 0.1-0.3% of the mass of the concentrated solution. At the same time, ultrasonic wave is applied to assist crystallization. In the third stage, the temperature is finally lowered to 10-15°C at 8-12°C / h to obtain a crystal slurry; S4. Pump the crystal slurry into a horizontal centrifuge to separate the crystals, and place the obtained crystals in a double-cone rotary dryer to dry them to a moisture content of ≤0.1%.

[0007] In the above technical solution, crude sodium borohydride uses an existing industrial-grade product with a sodium borohydride content of 85%-92% and water-insoluble matter ≤3 wt%; after S1 is filtered to obtain an extract containing sodium borohydride, membrane concentration is used instead of traditional evaporation concentration. After the extract enters the reverse osmosis system, high pressure is applied to the extract through the control system, forcing the extract to flow through the surface of the polypiperazineamide composite membrane. It should be noted that the surface of the polypiperazineamide composite membrane is covered with membrane pores, and the pore diameter of the pores is about 0.5 nm.

[0008] However, after S1 was stirred and filtered, the Fe in the solution was removed due to high temperature and high pressure dissolution. 3+ and Al 3+ , but the extract still contains water molecules, sodium borohydride molecules, Ca 2+ Mg 2+ and Na + Among them, the diameter of water molecules is 0.28nm. When passing through the polypiperazineamide composite membrane, it is precipitated by high pressure, Ca 2+ The diameter of the hydrated ions is 0.8-1.0nm, Mg 2+ The diameter of the hydrated ions is 0.8-0.95nm, Na +The hydrated ion diameter is 0.7-0.85nm. When the reverse osmosis system is under a pressure of 3-4 MPa and a membrane surface flow rate of 2-4 m / s, calcium ions with a hydrated diameter of 0.8-1.0 nanometers and magnesium ions with a hydrated diameter of 0.8-0.95 nanometers are physically blocked outside the membrane because their volume is much larger than the membrane pores. At the same time, the negatively charged carboxyl groups on the membrane surface produce strong electrostatic repulsion with the positively charged calcium and magnesium ions, continuously pushing the calcium ions and magnesium ions away from the membrane pores and retaining them in the circulation loop. In addition, the high-pressure pump continuously replenishes the extract. During the continuous supply of extract, water molecules are precipitated through the polypiperazineamide composite membrane, and the concentrations of calcium ions and magnesium ions in the circulation loop continue to rise. In the existing reverse osmosis system, the ion concentration in the liquid in the circulation loop can be monitored in real time. When the ion concentration reaches the threshold, the waste liquid discharge valve is automatically opened to discharge the high-concentration liquid. During the waste liquid discharge process, the single discharge volume is only 5-10% of the total circulating liquid.

[0009] In addition, in the extract, sodium borohydride is dissociated into Na + and borohydride anions. The hydrated diameter of borohydride anions is 0.34 nm, which can easily pass through the pores of the polypiperazineamide composite membrane and fall into the water production side. The water production side contains only water molecules, Na+ and borohydride anions, among which Na + Under the action of high voltage electric field in the membrane pore, Na + The hydrated water molecules are partially stripped off, and the effective diameter is reduced to 0.5nm. At the same time, the negative charge area of ​​the membrane pore wall induces the formation of local positive channels, which makes Na + They slide rapidly along the electrostatic potential well, and the high valence characteristics of calcium and magnesium ions make their hydration layer more stable, unable to be compressed and deformed under the same pressure. They are eventually strongly adsorbed and locked by the carboxyl and sulfonic acid groups in the pores, realizing the kinetic screening of ions of similar sizes, and retaining only water molecules, Na+ and borohydride anions on the water production side.

[0010] On this basis, the polypiperazineamide composite membrane used in S2 has a molecular weight cutoff of 200-300 Da. When the solute molecular weight is lower than 200 Da, its hydration diameter is about 0.5-0.6 nm, which can partially squeeze into the membrane pores and permeate. Solutes with a molecular weight of 200-300 Da form a hydration layer of 0.8-1.0 nm due to strong hydration, which is rigidly retained by the steric hindrance of the membrane pores; higher molecular weight substances are completely blocked.

[0011] In addition, in S3, the concentrated solution is transferred to a crystallization kettle, 0.005-0.02 wt% polyethylene glycol-4000 is added, and gradient cooling is performed. The ether oxygen groups of polyethylene glycol-4000 are selectively anchored to specific crystal planes of the sodium borohydride crystal nucleus through hydrogen bonds, forming a molecular barrier to inhibit the doping of impurity ions at lattice defect sites. Combined with gradient cooling, the supersaturation of the solution is precisely controlled to always be lower than the critical nucleation threshold, forcing the solute to grow layer by layer along the low-energy crystal plane, and finally obtaining sodium borohydride single crystals with uniform particle size and high purity, realizing the coordinated regulation of deep impurity removal and crystal morphology.

[0012] In another technical solution, in S3, gradient cooling is divided into three stages. In the second stage, after constant temperature, the temperature is lowered to 25-28°C at a rate of 4-6°C / h. When the temperature reaches 26±0.5°C, sodium borohydride seeds with a particle size of 50-100 μm are added. The amount of sodium borohydride seeds added is 0.1-0.3% of the mass of the concentrated solution. At the same time, ultrasound is applied to assist crystallization. The 50-100 μm seed crystals provide a lattice template that matches the sodium borohydride crystals, so that the solute preferentially grows epitaxially along its low-energy crystal plane, avoiding lattice defects caused by spontaneous nucleation. The simultaneously applied ultrasound generates microjets through the collapse of cavitation bubbles, instantly crushing the microcrystalline aggregates and forcing the solute molecules to migrate directionally to the seed crystal surface. At the same time, the ultrasonic shear force strips off the impurity ions adsorbed on the crystal surface, ultimately achieving crystal size monodispersity and purity improvement under a controlled low supersaturation environment.

[0013] Compared with the prior art, the present invention has the following beneficial effects: By replacing the traditional evaporation process with low-temperature concentration using a reverse osmosis membrane, impurity contamination with sodium metaborate is eradicated, eliminating the root cause of metal ion-induced dendrite growth. Polyethylene glycol-4000 is added during the crystallization process to inhibit the doping of impurity ions at lattice defect sites. At the same time, ultrasonic wave-assisted crystallization is applied during gradient cooling, allowing the solute to preferentially grow epitaxially along its low-energy crystal plane, avoiding lattice defects caused by spontaneous nucleation, and stripping impurity ions adsorbed on the crystal plane. In a controlled low-supersaturation environment, the monodispersity of crystal size and the improvement of purity are achieved. High-purity sodium borohydride crystals can be obtained in a single crystallization, thereby improving product yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of the process for preparing sodium borohydride in Example 1 of the present invention. DETAILED DESCRIPTION

[0015] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0016] At present, in the existing sodium borohydride purification process, evaporation and concentration lead to a high decomposition rate of sodium borohydride, and the residual sodium metaborate impurities in the concentrated solution induce the growth of crystal dendrites, forcing multiple recrystallizations and causing yield loss. The present invention provides a method for preparing sodium borohydride based on concentrated crystallization treatment, such as Figure 1 As shown, Example 1

[0017] Using crude sodium borohydride as raw material, wherein the crude sodium borohydride is an industrial-grade product with a sodium borohydride content of 88% and a water-insoluble matter content of 2.5 wt%, the method comprises the following steps: S1. Add crude sodium borohydride and deionized water in a mass ratio of 1:4 into an autoclave, stir at a constant temperature for 20 minutes, and filter to obtain an extract containing sodium borohydride; S2. Pumping the extract into a reverse osmosis system using a polypiperazineamide composite membrane, the reverse osmosis system concentrates the extract to a sodium borohydride mass concentration of ≥20% under the conditions of an operating pressure of 3.5 MPa and a membrane surface flow rate of 3 m / s; S3. The concentrated solution was transferred to a crystallization kettle, 0.01 wt% polyethylene glycol-4000 was added, and a gradient cooling process was performed in three stages: In the first stage, the crystallization kettle was kept at 40°C for 1.5 hours. In the second stage, the temperature was kept constant and then cooled down to 26°C at a rate of 5°C / h. When the temperature reached 26°C, sodium borohydride seeds with a particle size of 75 μm were added. The amount of sodium borohydride seeds added was 0.2% of the mass of the concentrated solution. Ultrasonic waves were applied to assist crystallization. In the third stage, the temperature was finally lowered to 13°C at 10°C / h to obtain a crystal slurry; S4. Pump the crystal slurry into a horizontal centrifuge to separate the crystals, and place the obtained crystals in a double-cone rotary dryer to dry them to a moisture content of 0.08%.

[0018] In the above preparation steps, it should be noted that the polypiperazineamide composite membrane is an existing technology and can be purchased from Bluestar Toray Membrane Technology (Beijing) Co., Ltd. The parameters are a molecular weight cutoff of 220-250Da and a maximum operating pressure of 5.5 MPa. The parameters of the autoclave in S1 are a temperature of 135°C, a pressure of 2.0 MPa, and a rotation speed of 200 rpm; the reverse osmosis system of S2 uses a four-stage series polypiperazineamide composite membrane; the ultrasonic parameters used in the second stage of S3 are a frequency of 20 kHz and a power density of 0.1 W / cm 3 , the action time is 10 minutes, the stirring rate of the crystallization kettle is 40 rpm, the centrifugal force of the S4 horizontal centrifuge is 900G, and the parameters of the double-cone rotary dryer are temperature 45°C, speed 8 rpm, and pressure -0.09 MPa. Example 2

[0019] S1. Crude sodium borohydride (85% sodium borohydride content) and deionized water were added into an autoclave at a mass ratio of 1:5. The autoclave parameters were as follows: temperature 120°C, pressure 1.5 MPa, speed 100 rpm, and constant temperature stirring for 30 minutes. S2. The extract is pumped into a reverse osmosis system using a three-stage series-connected polypiperazineamide composite membrane. The extract is concentrated to a sodium borohydride mass concentration of 20% under the conditions of an operating pressure of 3.0 MPa and a membrane surface velocity of 2 m / s. S3. Transfer the concentrated solution to a crystallization kettle, add 0.005 wt% polyethylene glycol-4000, and perform gradient cooling: In the first stage, the crystallization kettle was kept at 40°C for 2 hours. In the second stage, the temperature was kept constant and then cooled to 26°C at a rate of 4°C / h. When the temperature reached 26°C, sodium borohydride seeds with a particle size of 100 μm were added. The amount of sodium borohydride seeds added was 0.2% of the mass of the concentrated solution. At the same time, ultrasonic wave was applied to assist crystallization. The ultrasonic wave parameters were a frequency of 30 kHz and a power density of 0.05 W / cm 3 , action time 15 minutes, In the third stage, the temperature was finally lowered to 15°C at 8°C / h to obtain a crystal slurry; S4. The crystal slurry was pumped into a horizontal centrifuge to separate the crystals. The obtained crystals were placed in a double-cone rotary dryer and dried to a moisture content of 0.1%. The centrifugal force of the horizontal centrifuge was 800G. The parameters of the double-cone rotary dryer were temperature 40°C, rotation speed 5 rpm, and pressure -0.09 MPa. Example 3

[0020] S1. Crude sodium borohydride (92% sodium borohydride) and deionized water were added into an autoclave at a mass ratio of 1:3. The autoclave parameters were as follows: temperature 150°C, pressure 2.5 MPa, speed 300 rpm, and constant temperature stirring for 10 minutes. S2. The extract is pumped into a reverse osmosis system using a five-stage series-connected polypiperazineamide composite membrane. The extract is concentrated to a sodium borohydride concentration of 25% under the conditions of an operating pressure of 4.0 MPa and a membrane surface velocity of 4 m / s. S3. Transfer the concentrated solution to a crystallization kettle, add 0.02 wt% polyethylene glycol-4000, and perform gradient cooling: In the first stage, the crystallization kettle was kept at 40°C for 1 hour. In the second stage, the temperature was kept constant and then cooled to 25°C at a rate of 6°C / h. When the temperature reached 26°C, sodium borohydride seeds with a particle size of 50 μm were added. The amount of sodium borohydride seeds added was 0.2% of the mass of the concentrated solution. At the same time, ultrasonic wave was applied to assist crystallization. The ultrasonic wave parameters were a frequency of 10 kHz and a power density of 0.15 W / cm 3 , action time 5 minutes, In the third stage, the temperature was finally lowered to 10°C at 12°C / h to obtain a crystal slurry; S4. The crystal slurry is pumped into a horizontal centrifuge to separate the crystals, and the obtained crystals are placed in a double-cone rotary dryer and dried to a moisture content of 0.08%. The centrifugal force of the horizontal centrifuge is 1000G, and the parameters of the double-cone rotary dryer are temperature 50°C, speed 10 rpm, and pressure -0.09 MPa.

[0021] Comparative Example 1: Based on the steps of Example 1, the difference from Example 1 is that: In S2, the reverse osmosis system was eliminated and replaced with 80°C vacuum evaporation concentration. The basic principle of vacuum evaporation concentration is that when the vacuum system is maintained at -0.08 MPa, the boiling point of water drops to 62°C. At this time, the 80°C jacket heating causes the surface of the concentrate to rapidly vaporize. Water molecules are extracted in the form of vapor by the vacuum pump, while the solute, sodium metaborate, and impurity ions are retained in the liquid phase due to the difference in boiling points. However, this high temperature environment triggers the alkaline hydrolysis side reaction of sodium borohydride, generating sodium metaborate impurities and releasing hydrogen. In S3, polyethylene glycol-4000 was not added in the second stage of the gradient cooling.

[0022] Comparative Example 2: Based on the steps of Example 1, the difference from Example 1 is that: In S3, in the second stage of gradient cooling, polyethylene glycol-4000 was not added, and other step parameters were the same as in Example 1.

[0023] Comparative Example 3: Based on the steps of Example 1, the difference from Example 1 is that: In S3, in the second stage of gradient cooling, ultrasonic assisted crystallization is cancelled, and other step parameters are the same as those in Example 1.

[0024] Comparative Example 4: Based on the steps of Example 1, the difference from Example 1 is that: The polypiperazineamide composite membrane used in S2 is a monopolar configuration, the series cycle is eliminated, and the other step parameters are the same as in Example 1.

[0025] Comparative Example 5: Based on the steps of Example 1, the difference from Example 1 is that: In S3, the gradient cooling process is canceled and the temperature is directly set to drop from 40°C to 13°C at 10°C / h.

[0026] Comparative Example 6: Based on the steps of Example 1, the difference from Example 1 is that: The operating pressure of S2 is 5.0 MPa, and the rest is the same as in Example 1.

[0027] Comparative Example 7: Based on the steps of Example 1, the difference from Example 1 is that: In the crystallization kettle, in the second stage of gradient cooling in S3, the addition of sodium borohydride seed crystals was omitted, and the other procedures were the same as in Example 1.

[0028] Comparative Example 8: Based on the steps of Example 1, the difference from Example 1 is that: In S3, after the concentrated solution is transferred to the crystallization kettle, 0.01 wt% polyethylene glycol-2000 is added, and the rest is the same as in Example 1.

[0029] Comparative Example 9: The traditional recrystallization process is adopted, which specifically includes the following steps: 1. Take 100g of crude sodium borohydride (purity 92%, water-insoluble matter 2.8wt%), add 400mL of deionized water, stir and dissolve at 40℃ for 30min to obtain a clear solution; 2. Transfer the solution to a rotary evaporator and concentrate it at 85°C under a vacuum of -0.06 MPa to 25% of the original volume for 120 min. 3. The concentrated solution was transferred to a crystallization kettle, cooled naturally to 25°C at a rate of 1°C / min, allowed to stand for 2 hours, and centrifuged to obtain wet crystals; 4. Add 200 mL of methanol to the wet crystals and dissolve them under ultrasonication at 35°C for 10 min. Then perform a second concentration and remove 40% of the methanol by vacuum distillation at 60°C. The viscosity of the solution will increase significantly. Cool to -10°C, grow the crystals for 4 h, centrifuge and wash. Finally, repeat the above steps for three crystallizations. 5. The crystals were vacuum dried at 40°C under nitrogen atmosphere for 6 h to obtain refined sodium borohydride.

[0030] Experimental Example 1: In this experimental example, the purity of the sodium borohydride product was tested using the iodine titration method (GB / T 6284), the raw materials and products were weighed to calculate the yield, the unit energy consumption of each example was measured using an electric energy meter, and the aspect ratio of the sodium borohydride product crystals was statistically analyzed using SEM. 200 crystal particles were preferably selected for the statistical analysis.

[0031] Table 1: Experimental comparison of core performance of Examples 1 to 3 Test items Example 1 Example 2 Example 3 Product purity (%) 99.82 99.63 99.75 Yield (%) 95.3 93.1 94.7 Specific energy consumption (kWh / kg) 8.5 7.8 9.2 Crystal aspect ratio (μm) 1.02±0.05 1.08±0.10 1.05±0.08 Water insoluble residue (ppm) 120 185 95 Conclusion: Based on the experimental data of the core performance in Table 1, Example 1 is significantly ahead with a product purity of 99.82% and a yield of 95.3%. Its spherical crystal aspect ratio of 1.02±0.05μm and balanced energy consumption of 8.5 kWh / kg constitute the comprehensive optimal solution, and is the preferred preparation scheme for this scheme; while Example 2, with the lowest unit energy consumption of 7.8 kWh / kg, is suitable for working conditions with limited electricity or containing less than 85% low-purity raw materials; Example 3, relying on the lowest water-insoluble residue of 95 ppm, shows advantages in the deep purification of high-grade raw materials and production capacity priority scenarios.

[0032] Experimental Example 2: Concentration Process Comparison Experimental objects: Example 1, Comparative Example 1 and Comparative Example 9.

[0033] Table 2: Comparison of concentration processes Test items Example 1 Comparative Example 1 Comparative Example 9 Hydrolysis rate (%) 0.08 12.7 1.2 Product purity (%) 99.82 98.15 99.20 Yield (%) 95.3 88.2 75.6 Total metal ions (ppm) 8 35 18 Conclusion: As shown in the experimental data in Table 2, Example 1 fundamentally blocks the hydrolysis path of sodium borohydride through low-temperature membrane separation operation below 40°C, thereby avoiding the generation of a large amount of sodium metaborate impurities, and significantly reduces the decomposition of active ingredients caused by high temperature, so that the yield is increased to 95.3%. At the same time, the product purity and total metal ion amount indicators are higher than those of Comparative Example 1 and Comparative Example 9. Among them, Comparative Example 1 adopts 80°C vacuum evaporation concentration, and the hydrolysis rate is as high as 12.7%. At the same time, the product purity and total metal ion amount indicators are low standards. The yield loss of Comparative Example 9 due to multiple crystallizations in the traditional process is shown. Therefore, Example 1 is superior to Comparative Example 1 and Comparative Example 9.

[0034] Experimental Example 3: Additive Function Verification Experimental objects: Example 1, Comparative Example 2 and Comparative Example 8.

[0035] Table 3: Additive Function Verification Test items Example 1 Comparative Example 2 Comparative Example 8 Crystal aspect ratio (μm) 1.02±0.05 1.25±0.8 1.15±0.06 Product purity (%) 99.82 99.5 99.65 Yield (%) 95.3 88.2 91.0 Total metal ions (ppm) 8 25 15 Conclusion: As shown in the experimental data in Table 3, based on the same reverse osmosis process, Example 1, PEG-4000 suppresses the crystal aspect ratio to a nearly spherical shape through the long-chain steric hindrance effect, and its crystal aspect ratio is 1.02±0.05μm, and the product purity, yield and total metal ion amount are all better than Comparative Example 2 and Comparative Example 8. Comparative Example 2 has no additive polyethylene glycol-4000, and the crystal aspect ratio is 1.25±0.8μm, indicating the appearance of needle-shaped crystals. Comparative Example 8 replaces polyethylene glycol-4000 with polyethylene glycol-2000, and the crystal aspect ratio is 1.15±0.06μm; and the long-chain structure of polyethylene glycol-4000 used in Example 1 simultaneously exerts a metal chelating effect, suppressing the residual ions to 8 ppm. This verifies that Example 1 is better than Comparative Example 2 and Comparative Example 8.

[0036] Experimental Example 4: Reverse Osmosis System Optimization Verification Experimental objects: Example 1, Comparative Example 4 and Comparative Example 6.

[0037] Table 4: Optimization and verification of reverse osmosis system using series-connected polypiperazineamide composite membranes Test items Example 1 Comparative Example 4 Comparative Example 6 Water insoluble matter (ppm) 120 350 125 Membrane flux attenuation rate (% / h) 0.8 2.5 1.9 Calcium ion removal rate (%) 99.2 92.1 99.3 Conclusion: As shown in the experimental data in Table 4, the separation path is extended by connecting four membranes in series, so that the water-insoluble matter retention efficiency is increased to 120 ppm. At the same time, the membrane flux attenuation rate of 0.8% / h is significantly better than 2.5% / h of Comparative Example 4 and 1.9% / h of Comparative Example 6. In terms of calcium ion removal rate, 99.2% of Example 1 is significantly better than Comparative Example 4. Although it is slightly lower than 99.3% of Comparative Example 6, the other two experimental data also show that Example 1 is better than Comparative Example 4 and Comparative Example 6 in terms of the overall solution.

[0038] Experimental Example 5: Comprehensive Performance Comparison Experimental objects: Example 1, Comparative Example 3, Comparative Example 5 and Comparative Example 7.

[0039] Table 5: Comprehensive performance comparison Test items Example 1 Comparative Example 3 Comparative Example 5 Comparative Example 7 Product purity (%) 99.82 99.50 99.58 99.65 Crystal aspect ratio (μm) 1.02±0.05 1.25±0.06 1.10±0.05 1.15±0.08 Total metal ions (ppm) 8 25 15 15 Conclusion: As shown in the experimental data in Table 5, Example 1, through ultrasonic assistance combined with precise gradient cooling and the addition of sodium borohydride seeds, is superior to Comparative Examples 3, 5 and 7 in terms of product purity, crystal aspect ratio and total metal ion detection indicators. Among them, the direct cooling adopted in Comparative Example 5 destroys the growth dynamics, the crystals are ellipsoidal, the crystal aspect ratio is 1.10±0.05μm, the impurity removal rate decreases, and the total amount of metal ions remains at 15ppm, further verifying that Example 1 has high product purity, nearly spherical crystal morphology and high cleanliness, which is significantly better than Comparative Examples 3, 5 and 7.

[0040] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely preferred examples of the present invention and are not intended to limit the present invention. Various changes and improvements may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and improvements fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing sodium borohydride based on concentrated crystallization treatment, characterized in that: Using crude sodium borohydride as raw material, the process comprises the following steps: S1. Add crude sodium borohydride and deionized water in a mass ratio of 1:3-1:5 into an autoclave, stir at a constant temperature for 10-30 minutes, and filter to obtain an extract containing sodium borohydride; S2. Pumping the extract into a reverse osmosis system using a polypiperazineamide composite membrane to concentrate the extract to a sodium borohydride mass concentration of ≥20% under an operating pressure of 3-4 MPa and a membrane surface flow rate of 2-4 m / s; S3. The concentrated solution is transferred to a crystallization kettle, 0.005-0.02 wt% polyethylene glycol-4000 is added, and a gradient cooling process is performed in three stages: In the first stage, the crystallization kettle is kept at a constant temperature of 40°C for 1-2 hours. In the second stage, after constant temperature, the temperature is lowered to 25-28℃ at a rate of 4-6℃ / h. When the temperature reaches 26±0.5℃, sodium borohydride seeds with a particle size of 50-100 μm are added. The amount of sodium borohydride seeds added is 0.1-0.3% of the mass of the concentrated solution. At the same time, ultrasonic wave is applied to assist crystallization. In the third stage, the temperature is finally lowered to 10-15°C at 8-12°C / h to obtain a crystal slurry; S4. Pump the crystal slurry into a horizontal centrifuge to separate the crystals, and place the obtained crystals in a double-cone rotary dryer to dry them to a moisture content of ≤0.1%.

2. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, wherein: The crude sodium borohydride is an industrial-grade product with a sodium borohydride content of 85%-92% and water-insoluble matter ≤3 wt%.

3. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, wherein: In S1, the stirring parameters of the autoclave are temperature 120-150°C, pressure 1.5-2.5 MPa, and rotation speed 100-300 rpm.

4. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, wherein: The polypiperazineamide composite membrane used in S2 has a molecular weight cutoff of 200-300 Da.

5. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, wherein: The reverse osmosis system in S2 includes 3-5 stages of polypiperazineamide composite membrane units connected in series. The concentrated liquid in each stage is circulated for 2-4 times to remove metal ions in the concentrated liquid.

6. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, wherein: In S3, the ultrasonic frequency is 10-30kHz and the power density is 0.05-0.15 W / cm 3 , action time 5-15 minutes.

7. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, characterized in that: After the crystallization in S3 is completed, polyethylene glycol-4000 in the mother liquor is recovered by membrane filtration.

8. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, wherein: The double cone rotary dryer used in S4 has the following parameters: temperature 40-50°C, rotation speed 5-10 rpm, and pressure -0.09 MPa.

9. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, wherein: The stirring rate of the crystallization kettle in S3 was 20-60 rpm.

10. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, characterized in that: The centrifugal force of the S4 horizontal centrifuge is 800-1000 G.

Images