A method for preparing sodium borohydride based on concentration and crystallization treatment

Abstract

This invention relates to the field of sodium borohydride preparation technology, specifically to a method for preparing sodium borohydride based on concentration and crystallization. The method includes: adding crude sodium borohydride and deionized water to a high-pressure reactor in a specific mass ratio, stirring, and filtering to obtain an extract; pumping the extract into a reverse osmosis system for membrane concentration; transferring the concentrate to a crystallization reactor, adding polyethylene glycol-4000, and performing gradient cooling for crystallization, wherein sodium borohydride seed crystals are added in the second stage of gradient cooling while ultrasound is applied to assist crystallization; pumping the crystallized slurry into a horizontal centrifuge to separate the crystals, and placing the obtained crystals in a double-cone rotary dryer for drying to obtain concentrated sodium borohydride crystals; replacing the traditional evaporation process with low-temperature concentration via reverse osmosis membranes eliminates the byproduct sodium metaborate impurity contamination, removes the root cause of metal ion-induced dendrite growth, and allows for the acquisition of high-purity sodium borohydride crystals in a single crystallization, thus improving product yield.

Description

Technical Field

[0001] This invention relates to the field of sodium borohydride preparation technology, and more specifically, to a method for preparing sodium borohydride based on concentrated crystallization treatment. Background Technology

[0002] Sodium borohydride (NaBH4) is a versatile reducing agent for complex metal hydrides, widely used in both organic and inorganic chemistry. While its reducing power is weaker than lithium aluminum hydride for organic functional groups, it offers several unique advantages. For example, in the presence of 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 be used in alkaline aqueous solutions, ensuring safe operation—advantages unmatched by aluminum hydrides. Economically, sodium borohydride is less expensive than lithium aluminum hydride for the same reducing equivalent, thus enhancing its competitiveness as a commercial reducing agent for complex metal hydrides.

[0003] Traditional sodium borohydride preparation mainly employs the Schlesinger process and its modified techniques. Typically, metallic sodium, hydrogen, and trimethyl borate are used as raw materials. First, metallic sodium is melted and dispersed in mineral oil to form sodium sand. This sand reacts with hydrogen at 300-400℃ and 10-20 MPa to produce sodium hydride. Then, sodium hydride reacts with trimethyl borate in an inert solvent such as diethyl ether or tetrahydrofuran at 250-300℃ to produce sodium borohydride. The crude product is obtained through evaporation and concentration, cooling and crystallization, and filtration separation. Finally, it is purified through multiple recrystallizations.

[0004] In the preparation process of sodium borohydride, concentration and crystallization are key purification steps. In existing technologies, traditional concentration methods mainly rely on evaporation, but high-temperature conditions easily lead to the decomposition of sodium borohydride and increase the amount of sodium metaborate impurities, contaminating the crystallization system. In addition, the crystallization process often adopts single-stage cooling or simple gradient cooling, resulting in sodium borohydride crystals with a wide particle size distribution and easy agglomeration. Coupled with the residual sodium metaborate impurities, multiple recrystallizations are required to achieve high purity, resulting in yield loss. Summary of the Invention

[0005] This invention provides a method for preparing sodium borohydride based on concentration and crystallization. It replaces the traditional evaporation process with gradient cooling concentration using a reverse osmosis system, eliminating sodium metaborate impurities. Combined with a series of piperazine amide composite membranes for cyclic desalination, it eliminates the root cause of metal ion-induced dendrite growth. High-purity sodium borohydride crystals can be obtained in a single crystallization process, thus solving the problems mentioned in the background art.

[0006] 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 concentrate induce dendrite growth, forcing multiple recrystallizations and causing yield loss.

[0007] To achieve the above objectives, the method for preparing sodium borohydride includes the following steps:

[0008] S1. Add crude sodium borohydride and deionized water to a high-pressure reactor at a mass ratio of 1:3-1:5, stir at a constant temperature for 10-30 minutes, and filter to obtain an extract containing sodium borohydride.

[0009] S2. Pump the extract into a reverse osmosis system, which uses a polypiperazine amide composite membrane, and concentrate the extract to a sodium borohydride mass concentration ≥20% under operating pressure of 3-4 MPa and membrane surface flow rate of 2-4 m / s.

[0010] S3. Transfer the concentrate to a crystallization vessel, add 0.005-0.02 wt% polyethylene glycol-4000, and perform gradient cooling in three stages:

[0011] In the first stage, the crystallization vessel is kept at a constant temperature of 40°C for 1-2 hours.

[0012] In the second stage, after maintaining a 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 seed crystals with a particle size of 50-100 μm are added. The amount of sodium borohydride seed crystals added is 0.1-0.3% of the mass of the concentrated solution. At the same time, ultrasonic-assisted crystallization is applied.

[0013] The third and final stage involves cooling the material down to 10-15℃ at a rate of 8-12℃ / h to obtain a crystalline slurry.

[0014] S4. Pump the crystallized slurry into a horizontal centrifuge to separate the crystals. Place the resulting crystals in a double-cone rotary dryer and dry them until the moisture content is ≤0.1%.

[0015] In the above technical solution, the crude sodium borohydride uses existing industrial-grade products with a sodium borohydride content of 85%-92% and water-insoluble matter ≤3 wt%. After obtaining the sodium borohydride-containing extract through S1 filtration, membrane concentration is used instead of traditional evaporation concentration. Specifically, after the extract enters the reverse osmosis system, the control system generates high pressure on the extract, forcing it to flow through the surface of the polypiperazine amide composite membrane. It should be noted that the surface of the polypiperazine amide composite membrane is covered with membrane pores, and the pore diameter is approximately 0.5 nm.

[0016] However, although Fe was removed from the solution after stirring and filtration in S1 due to high temperature and high pressure dissolution, 3+ And Al 3+ However, the extract still contains water molecules, sodium borohydride molecules, and Ca2+ molecules.2+ Mg 2+ and Na + Among them, water molecules with a diameter of 0.28 nm are precipitated under high pressure when passing through the polypiperazine amide composite membrane, Ca 2+ The hydrated ions have a diameter of 0.8-1.0 nm, Mg 2+ The diameter of hydrated ions is 0.8-0.95 nm, Na + The hydrated ions have a diameter of 0.7-0.85 nm. Under reverse osmosis system conditions of 3-4 MPa pressure and 2-4 m / s flow rate at the membrane surface, calcium ions with a hydrated diameter of 0.8-1.0 nm and magnesium ions with a hydrated diameter of 0.8-0.95 nm are physically blocked outside the membrane due to their much larger volume than the membrane pores. At the same time, the negatively charged carboxyl groups on the membrane surface generate strong electrostatic repulsion with the positively charged calcium and magnesium ions, continuously pushing the calcium and magnesium ions away from the membrane pores and retaining them in the circulation loop. In addition, the high-pressure pump continuously replenishes the extractant. During the continuous supply of extractant, water molecules are precipitated through the polypiperazine amide composite membrane, and the concentration of calcium and magnesium ions in the circulation loop continues to rise. However, in existing reverse osmosis systems, 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 discharge volume in a single discharge is only 5-10% of the total circulating liquid volume.

[0017] In addition, sodium borohydride in the extract is dissociated into Na in water. + The borohydride anion, with a hydrated diameter of 0.34 nm, can easily pass through the pores of the polypiperazine amide composite membrane and fall into the product water side. The product water side contains only water molecules, Na+, and borohydride anions. + Under the influence of a high-voltage electric field within the membrane pores, Na + The hydrated water molecules are partially stripped away, reducing the effective diameter to 0.5 nm. Simultaneously, the negatively charged regions on the membrane pore walls induce the formation of localized positively charged channels, allowing Na... + The ions slide rapidly along the electrostatic potential trap, and the high valence of calcium and magnesium ions makes their hydration layer more stable, preventing them from being compressed and deformed under the same pressure. They are eventually strongly adsorbed and locked by carboxyl and sulfonic acid groups in the pores, achieving kinetic sieving of ions of similar size. Only water molecules, Na+ and boron hydride anions are retained on the water production side.

[0018] Based on this, the polypiperazine amide composite membrane used in S2 has a molecular weight cutoff of 200-300 Da. When the molecular weight of the solute is lower than 200 Da, its hydration diameter of about 0.5-0.6 nm can be partially squeezed 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 and are rigidly retained by the steric hindrance of the membrane pores. Substances with higher molecular weights are completely blocked.

[0019] In addition, in step S3, the concentrate is transferred to a crystallization vessel, and 0.005-0.02 wt% polyethylene glycol-4000 is added. Gradient cooling is then performed. The ether oxygen groups of polyethylene glycol-4000 are selectively anchored to specific crystal planes of sodium borohydride crystal nuclei through hydrogen bonds, forming a molecular barrier that inhibits the doping of impurity ions at lattice defect sites. Combined with gradient cooling, the supersaturation of the solution is precisely controlled to always be below the critical nucleation threshold, forcing the solute to grow layer by layer along low-energy crystal planes. Finally, uniform and high-purity sodium borohydride single crystals are obtained, achieving synergistic control of impurity removal and crystal morphology.

[0020] In another technical solution, in S3, the gradient cooling is divided into three stages. In the second stage, after maintaining a constant temperature, the temperature is reduced to 25-28℃ at a rate of 4-6℃ / h. When the temperature reaches 26±0.5℃, sodium borohydride seed crystals with a particle size of 50-100 μm are added. The amount of sodium borohydride seed crystals added is 0.1-0.3% of the mass of the concentrated solution. At the same time, ultrasonic-assisted crystallization is applied. The 50-100 μm seed crystals provide a lattice template that matches the sodium borohydride crystals, allowing the solute to preferentially grow epitaxially along its low-energy crystal plane, avoiding lattice defects caused by spontaneous nucleation. The simultaneously applied ultrasonic waves instantly pulverize the microcrystal aggregates through the microjets generated by the collapse of cavitation bubbles, forcing solute molecules to migrate directionally to the surface of the seed crystals. At the same time, the ultrasonic shear force strips the impurity ions adsorbed on the crystal plane. Finally, under a controlled low supersaturation environment, the monodispersity of crystal size and the improvement of purity are achieved.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] By replacing the traditional evaporation process with low-temperature concentration using reverse osmosis membranes, sodium metaborate impurity contamination is eliminated, removing the root cause of metal ion-induced dendrite growth. Polyethylene glycol-4000 is added during the crystallization process to suppress the doping of impurity ions at lattice defect sites. Simultaneously, ultrasonic-assisted crystallization is applied during gradient cooling, causing 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. Under a controlled low supersaturation environment, monodispersity and purity of crystal size are improved. High-purity sodium borohydride crystals can be obtained in a single crystallization, increasing product yield. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the sodium borohydride preparation method in Example 1 of the present invention. Detailed Implementation

[0024] 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.

[0025] Currently, existing sodium borohydride purification processes suffer from high sodium borohydride decomposition rates due to evaporation and concentration, and the residual sodium metaborate impurities in the concentrated solution induce dendrite growth, forcing multiple recrystallizations and resulting in yield losses. This invention provides a method for preparing sodium borohydride based on concentration and crystallization treatment, such as... Figure 1 As shown, Example 1

[0026] 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 water-insoluble matter of 2.5 wt%, the process includes the following steps:

[0027] S1. Add crude sodium borohydride and deionized water to a high-pressure reactor at a mass ratio of 1:4, stir at a constant temperature for 20 minutes, and filter to obtain an extract containing sodium borohydride.

[0028] S2. The extract is pumped into a reverse osmosis system, which uses a polypiperazine amide composite membrane. The extract is concentrated to a sodium borohydride mass concentration of ≥20% under the conditions of operating pressure of 3.5 MPa and membrane surface flow rate of 3 m / s.

[0029] S3. Transfer the concentrate to a crystallization vessel, add 0.01 wt% polyethylene glycol-4000, and perform gradient cooling in three stages:

[0030] In the first stage, the crystallization vessel is kept at a constant temperature of 40°C for 1.5 hours.

[0031] In the second stage, after maintaining a constant temperature, the temperature is lowered to 26℃ at a rate of 5℃ / h. When the temperature reaches 26℃, sodium borohydride seed crystals with a particle size of 75 μm are added. The amount of sodium borohydride seed crystals added is 0.2% of the mass of the concentrated solution. At the same time, ultrasonic-assisted crystallization is applied.

[0032] The third and final stage involves cooling the material down to 13°C at a rate of 10°C / h to obtain a crystalline slurry.

[0033] S4. Pump the crystallized slurry into a horizontal centrifuge to separate the crystals. Place the resulting crystals in a double-cone rotary dryer and dry them to a moisture content of 0.08%.

[0034] In the above preparation steps, it should be noted that the polypiperazine amide composite membrane is existing technology and can be purchased from Bluestar Toray Membrane Technology (Beijing) Co., Ltd. Its parameters are: molecular weight cutoff 220-250 Da, maximum operating pressure 5.5 MPa; the parameters of the autoclave in S1 are: temperature 135℃, pressure 2.0 MPa, rotation speed 200 rpm; the reverse osmosis system in S2 uses a 4-stage series polypiperazine amide composite membrane; the ultrasonic parameters used in the second stage of S3 are: frequency 20 kHz, power density 0.1 W / cm³. 3 The reaction time was 10 minutes, the stirring rate of the crystallization vessel was 40 rpm, the centrifugal force of the S4 horizontal centrifuge was 900G, and the parameters of the double cone rotary dryer were 45℃ temperature, 8 rpm speed, and -0.09 MPa pressure. Example 2

[0035] S1. Add crude sodium borohydride (85% sodium borohydride content) and deionized water to a high-pressure reactor at a mass ratio of 1:5. The parameters of the high-pressure reactor are: temperature 120℃, pressure 1.5MPa, speed 100rpm, and constant temperature stirring for 30 minutes.

[0036] S2. Pump the extract into the reverse osmosis system, which uses a 3-stage series polypiperazine amide composite membrane. Concentrate the extract to a sodium borohydride mass concentration of 20% under the conditions of operating pressure of 3.0 MPa and membrane surface flow rate of 2 m / s.

[0037] S3. Transfer the concentrate to a crystallization vessel, add 0.005 wt% polyethylene glycol-4000, and perform gradient cooling:

[0038] In the first stage, the crystallization vessel is kept at a constant temperature of 40°C for 2 hours.

[0039] In the second stage, after maintaining a constant temperature, the temperature is lowered to 26℃ at a rate of 4℃ / h. Upon reaching 26℃, sodium borohydride seed crystals with a particle size of 100μm are added at a concentration of 0.2% of the concentrated solution mass. Simultaneously, ultrasonic-assisted crystallization is applied with a frequency of 30kHz and a power density of 0.05W / cm³. 3 The effect lasts for 15 minutes.

[0040] The third and final stage involves cooling the material down to 15°C at a rate of 8°C / h to obtain a crystalline slurry.

[0041] S4. Pump the crystallized slurry into a horizontal centrifuge to separate the crystals. Place the resulting crystals in a double cone rotary dryer and dry them to a moisture content of 0.1%. The centrifugal force of the horizontal centrifuge is 800G, and the parameters of the double cone rotary dryer are: temperature 40℃, speed 5 rpm, and pressure -0.09 MPa. Example 3

[0042] S1. Add crude sodium borohydride (sodium borohydride content 92%) and deionized water to a high-pressure reactor at a mass ratio of 1:3. The parameters of the high-pressure reactor are: temperature 150℃, pressure 2.5MPa, speed 300rpm, and constant temperature stirring for 10 minutes.

[0043] S2. Pump the extract into the reverse osmosis system, which uses a 5-stage series polypiperazine amide composite membrane. Concentrate the extract to a sodium borohydride mass concentration of 25% under the conditions of operating pressure of 4.0 MPa and membrane surface flow rate of 4 m / s.

[0044] S3. Transfer the concentrate to a crystallization vessel, add 0.02 wt% polyethylene glycol-4000, and perform gradient cooling:

[0045] In the first stage, the crystallization vessel is kept at a constant temperature of 40°C for 1 hour.

[0046] In the second stage, after maintaining a constant temperature, the temperature is lowered to 25℃ at a rate of 6℃ / h. When the temperature reaches 26℃, sodium borohydride seed crystals with a particle size of 50μm are added. The amount of sodium borohydride seed crystals added is 0.2% of the mass of the concentrated solution. At the same time, ultrasonic-assisted crystallization is applied with the following parameters: frequency 10kHz, power density 0.15W / cm³. 3 The effect lasts for 5 minutes.

[0047] The third and final stage involves cooling the material down to 10°C at a rate of 12°C / h to obtain a crystalline slurry.

[0048] S4. Pump the crystallized slurry into a horizontal centrifuge to separate the crystals. Place the resulting crystals in a double cone rotary dryer and dry them 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℃, speed 10rpm, and pressure -0.09 MPa.

[0049] Comparative Example 1:

[0050] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0051] In S2, the reverse osmosis system is removed and replaced with 80℃ vacuum evaporation concentration. The basic principle of vacuum evaporation concentration is that under the condition of maintaining a vacuum system of -0.08 MPa, the boiling point of water drops to 62℃. At this time, the jacket heating at 80℃ causes the surface of the concentrate to vaporize rapidly. Water molecules are extracted by the vacuum pump in the form of vapor, while the solute, sodium metaborate and impurity ions remain in the liquid phase due to the difference in boiling points. However, this high-temperature environment will trigger the alkaline hydrolysis side reaction of sodium borohydride, generating sodium metaborate impurities and releasing hydrogen gas.

[0052] In S3, polyethylene glycol-4000 is not added during the second stage of gradient cooling.

[0053] Comparative Example 2:

[0054] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0055] In S3, during the second stage of gradient cooling, polyethylene glycol-4000 is not added, and other steps and parameters are the same as in Example 1.

[0056] Comparative Example 3:

[0057] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0058] In S3, during the second stage of gradient cooling, ultrasonic-assisted crystallization is cancelled, while other steps and parameters are the same as in Example 1.

[0059] Comparative Example 4:

[0060] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0061] The polypiperazine amide composite membrane used in S2 is a unipolar setup, and the series circulation is cancelled. Other steps and parameters are the same as in Example 1.

[0062] Comparative Example 5:

[0063] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0064] In S3, the gradient cooling process is canceled, and it is directly set to cool from 40℃ to 13℃ at a rate of 10℃ / h.

[0065] Comparative Example 6:

[0066] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0067] S2 operating pressure is 5.0 MPa, and other parameters are the same as in Example 1.

[0068] Comparative Example 7:

[0069] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0070] In S3, during the second stage of gradient cooling in the crystallization vessel, the addition of sodium borohydride seed crystals was omitted, while the rest was the same as in Example 1.

[0071] Comparative Example 8:

[0072] Based on the steps of Example 1, the difference from Example 1 is as follows:

[0073] In S3, after transferring the concentrate to the crystallization vessel, the additive is 0.01 wt% polyethylene glycol-2000, and the rest is the same as in Example 1.

[0074] Comparative Example 9:

[0075] The traditional recrystallization process includes the following steps:

[0076] 1. Take 100g of crude sodium borohydride (purity 92%, water-insoluble matter 2.8wt%), add 400mL of deionized water, stir at 40℃ for 30min to dissolve, and obtain a clear solution;

[0077] 2. The solution was transferred to a rotary evaporator and concentrated to 25% of its original volume at 85°C under a vacuum of -0.06 MPa for 120 minutes.

[0078] 3. The concentrated solution was transferred to a crystallization vessel and naturally cooled to 25°C at a rate of 1°C / min. After standing and aging for 2 hours, it was centrifuged to obtain wet crystals.

[0079] 4. Add 200 mL of methanol to the wet crystals and sonicate at 35 °C for 10 min. Then concentrate the solution twice and remove 40% of the methanol by vacuum distillation at 60 °C. The solution viscosity increases significantly. Cool the solution to -10 °C and grow crystals for 4 h. Centrifuge and wash the crystals. Finally, repeat the above steps three times for crystallization.

[0080] 5. The crystals were dried under a nitrogen atmosphere at 40°C for 6 hours to obtain refined sodium borohydride.

[0081] Experimental Example 1:

[0082] In this experimental example, the purity of the sodium borohydride product was determined by iodometric titration (GB / T 6284), the yield was calculated by material weighing of the raw materials and products, the unit energy consumption of each embodiment was measured by an electricity meter, and the aspect ratio of the sodium borohydride product crystals was statistically analyzed using SEM. The optimal number of crystal particles was 200.

[0083] Table 1: Experimental Comparison of Core Performance in Examples 1 to 3

[0084] Test items Example 1 Example 2 Example 3 Product purity (%) 99.82 99.63 99.75 Yield (%) 95.3 93.1 94.7 Energy consumption per unit (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 matter residue (ppm) 120 185 95

[0085] Conclusion: Based on the experimental data of the core performance in Table 1, Example 1 significantly leads 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 optimal solution, making it the preferred preparation scheme adopted in this plan. Example 2, with the lowest unit energy consumption of 7.8 kWh / kg, is suitable for working conditions with limited power or containing less than 85% low-purity raw materials. Example 3, relying on the lowest water-insoluble matter residue of 95 ppm, demonstrates its advantages in scenarios of deep purification of high-grade raw materials and priority of production capacity.

[0086] Experimental Example 2: Comparison of Concentration Processes

[0087] Experimental subjects: Example 1, Comparative Example 1 and Comparative Example 9.

[0088] Table 2: Comparison of Concentration Processes

[0089] 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

[0090] Conclusion: As shown in Table 2, Example 1, through low-temperature membrane separation below 40°C, fundamentally blocked the hydrolysis pathway of sodium borohydride, thus avoiding the generation of a large amount of sodium metaborate impurities. It also significantly reduced the decomposition of active ingredients caused by high temperatures, increasing the yield to 95.3%. Furthermore, the product purity and total metal ion content were higher than those of Comparative Example 1 and Comparative Example 9. Comparative Example 1, due to its 80°C vacuum evaporation concentration, had a hydrolysis rate as high as 12.7%, and its product purity and total metal ion content were below standard. Comparative Example 9, using a traditional process, suffered yield losses due to multiple crystallizations. Therefore, Example 1 is superior to Comparative Example 1 and Comparative Example 9.

[0091] Experiment Example 3: Functional Verification of Additives

[0092] Experimental subjects: Example 1, Comparative Example 2 and Comparative Example 8.

[0093] Table 3: Functional Validation of Additives

[0094] 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

[0095] Conclusion: As shown in Table 3, based on the same reverse osmosis process, Example 1, through the steric hindrance effect of its long chain, compressed the crystal aspect ratio to near spherical shape, with an aspect ratio of 1.02±0.05μm. Furthermore, Example 1 outperformed Comparative Examples 2 and 8 in terms of product purity, yield, and total metal ion content. Comparative Example 2, lacking the additive polyethylene glycol-4000, exhibited a crystal aspect ratio of 1.25±0.8μm, indicating needle-like crystal formation. Comparative Example 8, replacing polyethylene glycol-4000 with polyethylene glycol-2000, had a crystal aspect ratio of 1.15±0.06μm. Moreover, the long-chain structure of polyethylene glycol-4000 used in Example 1 simultaneously exerted a metal chelating effect, suppressing residual ions to 8 ppm. This verifies that Example 1 is superior to Comparative Examples 2 and 8.

[0096] Experiment Example 4: Optimization and Verification of Reverse Osmosis System

[0097] Experimental subjects: Example 1, Comparative Example 4 and Comparative Example 6.

[0098] Table 4: Optimization and Validation of Reverse Osmosis System Using Tandem Polypiperazine Amide Composite Membrane

[0099] Test items Example 1 Comparative Example 4 Comparative Example 6 Water-insoluble matter (ppm) 120 350 125 Membrane flux decay rate (% / h) 0.8 2.5 1.9 Calcium ion removal rate (%) 99.2 92.1 99.3

[0100] Conclusion: As shown in Table 4, by extending the separation path through a 4-stage membrane series connection, the water-insoluble matter rejection efficiency was increased to 120 ppm. Meanwhile, the membrane flux decay rate of 0.8% / h was significantly better than that of Comparative Example 4 (2.5% / h) and Comparative Example 6 (1.9% / h). Regarding calcium ion removal rate, Example 1's 99.2% was significantly better than Comparative Example 4, although slightly lower than Comparative Example 6's 99.3%. However, the other two experimental data also demonstrate that Example 1's overall approach is superior to Comparative Examples 4 and 6.

[0101] Experiment Example 5: Overall Performance Comparison

[0102] Experimental subjects: Example 1, Comparative Example 3, Comparative Example 5 and Comparative Example 7.

[0103] Table 5: Overall Performance Comparison

[0104] 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

[0105] Conclusion: As shown in Table 5, Example 1, through ultrasonic assistance combined with precise gradient cooling and the addition of sodium borohydride seed crystals, outperformed Comparative Examples 3, 5, and 7 in terms of product purity, crystal aspect ratio, and total metal ion content. Comparative Example 5, which used direct cooling to disrupt growth kinetics, resulted in ellipsoidal crystals with an aspect ratio of 1.10 ± 0.05 μm, a decreased impurity removal rate, and a residual total metal ion content of 15 ppm. This further verifies that Example 1 simultaneously possesses high product purity, near-spherical crystal morphology, and high cleanliness, significantly superior to Comparative Examples 3, 5, and 7.

[0106] The foregoing has shown and described 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 embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of 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 includes the following steps: S1. Add crude sodium borohydride and deionized water to a high-pressure reactor at a mass ratio of 1:3-1:5, stir at a constant temperature for 10-30 minutes, and filter to obtain an extract containing sodium borohydride. S2. Pump the extract into a reverse osmosis system, which uses a polypiperazine amide composite membrane, and concentrate the extract to a sodium borohydride mass concentration ≥20% under operating pressure of 3-4 MPa and membrane surface flow rate of 2-4 m / s. S3. Transfer the concentrate to a crystallization vessel, add 0.005-0.02 wt% polyethylene glycol-4000, and perform gradient cooling in three stages: In the first stage, the crystallization vessel is kept at a constant temperature of 40°C for 1-2 hours. In the second stage, after maintaining a 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 seed crystals with a particle size of 50-100 μm are added. The amount of sodium borohydride seed crystals added is 0.1-0.3% of the mass of the concentrated solution. At the same time, ultrasonic-assisted crystallization is applied. The third and final stage involves cooling the material down to 10-15℃ at a rate of 8-12℃ / h to obtain a crystalline slurry. S4. Pump the crystallized slurry into a horizontal centrifuge to separate the crystals. Place the resulting crystals in a double-cone rotary dryer and dry them until the moisture content is ≤0.1%.

2. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, characterized in that: 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, characterized in that: In S1, the stirring parameters of the autoclave are: temperature 120-150℃, 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, characterized in that: The polypiperazine amide 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, characterized in that: The reverse osmosis system in S2 contains 3-5 stages of polypiperazine amide composite membrane units connected in series. Each stage of concentrate is circulated 2-4 times to remove metal ions from the concentrate.

6. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, characterized in that: In S3, the ultrasonic frequency is 10-30kHz, and the power density is 0.05-0.15 W / cm². 3 The effect lasts for 5-15 minutes.

7. The method for preparing sodium borohydride based on concentrated crystallization treatment according to claim 1, characterized in that: After 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, characterized in that: The double cone rotary dryer used in S4 has the following parameters: temperature 40-50℃, 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, characterized in that: The stirring speed of the crystallizer in S3 is 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 horizontal centrifuge in S4 is 800-1000 G.

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