Preparation and morphology control process of high-purity sodium fluoride

Abstract

The application discloses a preparation and morphology control process of high-purity sodium fluoride, which comprises the following steps: preparing an initial bottom solution containing sodium fluorosilicate and a high-molecular dispersant; under stirring, sodium carbonate solution is added into the initial bottom solution, the feeding rate of the sodium carbonate solution is controlled in stages to realize gradient adjustment of the pH value of the system, a crystallization reaction is completed, and after the pH value of the system is stable, a mixed slurry containing sodium fluoride crystals and dispersed silicon dioxide is obtained; the mixed slurry is introduced into a hydrocyclone for classification and separation, and sodium fluoride crude products are collected through underflow; and the sodium fluoride crude products are subjected to purification treatment to obtain high-purity sodium fluoride products. The application is novel, and through the synergistic effect of silicon removal, stage-by-stage crystallization control and metal-free mother liquor circulation, the purity and separation problem caused by by-product silicon dioxide can be effectively overcome, the purity and comprehensive yield of high-purity sodium fluoride are greatly improved, and the application has good industrial application value.

Description

Technical Field

[0001] This invention relates to the field of sodium fluoride preparation technology, and in particular to a process for preparing high-purity sodium fluoride and controlling its morphology. Background Technology

[0002] Sodium fluoride ( ) as a preparation of sodium hexafluorophosphate ( Sodium fluoride is a core precursor material for sodium-ion battery electrolytes, as well as an important raw material for high-end optical materials and metallurgical fluxes. Its purity, impurity content (especially silicon content and metal ions), and crystal morphology directly determine the electrochemical performance and optical stability of downstream products. With the rapid iteration of emerging industries such as sodium-ion batteries, the industry has put forward extremely stringent requirements for the physicochemical properties of battery-grade high-purity sodium fluoride.

[0003] While existing processes for preparing sodium fluoride using the reaction of sodium fluorosilicate and sodium carbonate are low-cost, the reaction byproducts (amorphous silica gel) readily co-precipitate with the nascent sodium fluoride microcrystals, tightly encapsulating them and leading to excessive silicon impurities in the product. Simultaneously, traditional crystallization processes often produce fine, agglomerated microcrystals with internally closed capillary pores, easily resulting in the entrainment of mother liquor and impurities. Furthermore, the viscous silica gel makes solid-liquid separation and washing extremely difficult, not only hindering the achievement of the maximum purity limit but also causing low overall production yield, rapid filter material consumption, and poor economic efficiency. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies, such as low product purity limits, low overall production yield, rapid filter material consumption, and poor economic benefits, and to propose a process for the preparation and morphology control of high-purity sodium fluoride.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A process for preparing and controlling the morphology of high-purity sodium fluoride includes the following steps:

[0007] S1. Prepare the initial base solution containing sodium fluorosilicate and a polymeric dispersant;

[0008] S2. Under stirring, sodium carbonate solution is added to the initial base liquid. The pH value of the system is adjusted by controlling the feeding rate of sodium carbonate solution in stages to complete the crystallization reaction. After the pH of the system stabilizes, a mixed slurry containing sodium fluoride crystals and dispersed silica is obtained.

[0009] S3. The mixed slurry is introduced into a hydrocyclone for classification and separation. The crude sodium fluoride is collected by the underflow and the mother liquor containing silica is obtained by the overflow.

[0010] S4. The crude sodium fluoride product is purified to obtain high-purity sodium fluoride product.

[0011] S5. The overflowing silica-containing mother liquor is subjected to desiliconization solid-liquid separation to obtain desiliconized mother liquor. The desiliconized mother liquor is divided into a circulating stream and a waste stream in proportion. The circulating stream is returned to step S1 or step S2 for recycling.

[0012] Preferably, in step S1, the polymeric dispersant is selected from at least one of polyethylene glycol, polyacrylamide, sodium polyacrylate, and polyvinyl alcohol;

[0013] The amount of the polymeric dispersant added is 0.01% to 1.0% of the mass of sodium fluorosilicate.

[0014] Preferably, before adding sodium carbonate solution in step S2, the method further includes a step of pre-adding ultrafine high-purity sodium fluoride seed crystals to the initial base solution;

[0015] The average particle size D50 of the sodium fluoride seed crystals is controlled between 1 and 5 μm, and the sodium fluoride seed crystals are added to the system in the form of a pre-dispersed aqueous suspension.

[0016] The amount of seed crystals added is 0.5% to 1.0% of the theoretical mass of sodium fluoride produced.

[0017] Preferably, in step S2, the sodium carbonate solution is injected into the reaction system through a pipeline static mixer or a high-shear emulsifier, so that the added sodium carbonate solution and the initial base liquid achieve high-shear dispersion and mixing at the moment of injection.

[0018] Preferably, in step S2, the crystallization reaction temperature is controlled at 85–90°C; the step-by-step control of the sodium carbonate solution feeding rate to achieve gradient adjustment of the system pH value specifically includes:

[0019] Rapid nucleation stage: Sodium carbonate solution is added to the system at the first feeding rate. During this stage, the pH value of the system rises from the initial value to 6.5-7.5.

[0020] Crystal growth stage: Reduce the stirring speed of the system and continue to add sodium carbonate solution at a second feeding rate that is less than the first feeding rate until the pH value of the system reaches the reaction endpoint of 8.5 to 9.0.

[0021] Preferably, the second feeding rate is set to 10% to 20% of the first feeding rate.

[0022] Preferably, in step S4, the purification process includes washing and drying; wherein the washing is performed using pure water in a countercurrent manner; and the drying is performed using a fluidized bed drying system under hot air conditions at 100–120°C.

[0023] Preferably, in step S5, the specific operation of the desiliconization solid-liquid separation is as follows: a flocculant is added to the silica-containing mother liquor to cause the suspended silica to coagulate and settle, and the desiliconized mother liquor is obtained after pressure filtration separation;

[0024] The flocculant is a metal ion-free organic polymer flocculant.

[0025] Preferably, in step S5, 80% to 85% of the total amount of the desilication mother liquor is returned to the recycling system, and the remaining 15% to 20% is discharged from the waste system.

[0026] Preferably, in the entire process system, the inner surface material or coating of all reaction devices, agitators, hydrocyclones and conveying pipelines that come into contact with fluorine-containing materials are polytetrafluoroethylene or perfluoroalkoxy resin.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] 1. In this invention, by introducing a polymeric dispersant into the base liquid, the encapsulation and co-precipitation of sodium fluoride crystals by the byproduct silica is effectively blocked. At the same time, combined with the efficient centrifugal classification of the hydrocyclone, the process defect of easy clogging of silica gel filters is solved, and the silicon content in the product is significantly reduced.

[0029] 2. In this invention, a crystallization process involving seed crystal guidance and staged gradient feeding is used to induce sodium fluoride to achieve dense, regular, and large-particle-size layered growth. This morphology control eliminates capillary pores and mother liquor entrainment caused by fragmented crystal agglomeration, significantly reducing washing difficulty and further improving the upper limit of product purity.

[0030] 3. In this invention, an organic polymer flocculant without metal ions is used in the mother liquor treatment, which avoids the exogenous metal pollution of traditional processes; combined with the circular design of proportional open-loop waste discharge, the comprehensive fluorine yield is stably increased to more than 90% under the premise of preventing the malignant enrichment of impurities in the system, which greatly improves the economic benefits of large-scale long-cycle production.

[0031] This invention features a novel design that effectively overcomes the purity and separation challenges posed by the byproduct silica through the synergistic effect of dispersed silicon removal, staged crystallization control, and metal-free mother liquor circulation. This significantly improves the purity and overall yield of high-purity sodium fluoride and has excellent industrial application value. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0033] Description of Experimental Materials and Equipment

[0034] In the embodiments and comparative examples of this invention, all equipment that comes into contact with fluorine-containing materials (including the inner wall of a 10L stainless steel reactor, mechanical stirring paddle, hydrocyclone lining, pipelines and trays, etc.) are strictly made of polytetrafluoroethylene (PTFE) or perfluoroalkoxy resin (PFA) or coating. This effectively avoids the interference of the leaching of metal ions and silicon ions on the purity of the product under alternating acid and alkali conditions.

[0035] The main parameters of the raw materials and reagents used are as follows:

[0036] Sodium fluorosilicate: purity ≥98%, industrial grade;

[0037] Sodium carbonate: analytical grade;

[0038] Water: Pure water or deionized water;

[0039] High-purity sodium fluoride seed crystals: main content ≥99.9%.

[0040] Example 1

[0041] Step S1: Prepare the initial base solution

[0042] Add 3 kg of deionized water to a 10 L reactor and start mechanical stirring (250 rpm). Add 1 kg of sodium fluorosilicate powder (purity ≥98%, 200 mesh) and stir until a suspension is formed. Then add 0.5 g of sodium polyacrylate (molecular weight approximately 8000, equivalent to 0.05% of the mass of sodium fluorosilicate) as a polymeric dispersant and continue stirring for 15 minutes to ensure complete dissolution and adsorption onto the surface of the sodium fluorosilicate particles. The pre-coating effect of the dispersant prevents the subsequent reaction byproducts (amorphous silica gel) from encapsulating the newly formed nuclei, laying the foundation for the crystallization of high-purity sodium fluoride.

[0043] Step S2: Staged crystallization reaction

[0044] The above-mentioned base liquid was heated to 88°C, and the stirring speed was maintained at 250 rpm. The saturated sodium carbonate solution preheated to 80°C was added to the reaction vessel by a peristaltic pump.

[0045] The core chemical reaction that occurs in this process is:

[0046]

[0047] The feeding process is divided into two stages:

[0048] Rapid nucleation stage: Saturated sodium carbonate solution is added at a first feeding rate of 110 g / min. During this stage, a large number of bubbles can be observed in the reaction solution. The system continuously generates pH, which is monitored in real time with an online pH meter. The pH value of the system gradually increases from the initial value of about 4.5. After about 40 minutes, the pH value reaches about 7.0, and the system enters the next stage. The large amount of carbon dioxide microbubbles released in the reaction solution also form micro-airflow stirring, tearing apart large-volume aggregates and improving the uniformity of crystal nucleus distribution.

[0049] Crystal growth stage: The stirring speed was reduced to 150 rpm, and the feeding rate of sodium carbonate solution was reduced to 18 g / min (approximately 16% of the first feeding rate). The addition continued slowly, and the bubble generation rate decreased significantly. After approximately 65 minutes, no more bubbles were generated, the pH stabilized at 8.8, and the reaction reached its endpoint. During the crystal growth stage, newly generated molecules preferentially deposited on the surface of the crystal nuclei formed in the first stage, undergoing layered growth. Combined with the lower stirring shear force, this induced sodium fluoride to develop into dense, regular, large-particle crystals, eliminating capillary pores and mother liquor entrainment caused by the aggregation of small fragments.

[0050] Approximately 5.6 kg of saturated sodium carbonate solution was added throughout the process, and the reaction temperature was maintained at 86–89 °C. The final product was a mixed slurry containing sodium fluoride crystals and dispersed silica.

[0051] Step S3: Hydrocyclone Classification and Separation

[0052] The mixed slurry (temperature approximately 88°C) after the reaction was completed was pumped into a laboratory hydrocyclone (25 mm in diameter, 8° cone angle) at a feed pressure of 0.25 MPa. The underflow was collected to obtain a crude sodium fluoride slurry with a solid content of approximately 55%, and the overflow was collected to obtain a mother liquor containing silica.

[0053] Step S4: Purification

[0054] The underflow slurry was filtered using a Buchner funnel (equipped with polypropylene filter cloth) to obtain a wet filter cake. The cake was then washed countercurrently with deionized water: the slurry was adjusted to a solid-liquid ratio of 1:2, stirred for 10 minutes, and then filtered. This process was repeated twice. Finally, the wet material was placed in a fluidized bed dryer and dried at 110°C for 6 hours until constant weight was achieved, yielding a high-purity sodium fluoride product.

[0055] Step S5: Mother liquor treatment and open-loop circulation

[0056] The overflowing silica-containing mother liquor was collected, and 0.05 g of anionic polyacrylamide (APAM) was added as a flocculant. The mixture was slowly stirred for 5 minutes and then allowed to settle for 2 hours. The supernatant was vacuum filtered to obtain a clarified desiliconized mother liquor. The total volume of the desiliconized mother liquor was divided into two streams: 85% was used as a circulating stream, returned to step S1 to replace deionized water for preparing the initial bottom solution for the next batch; 15% was discharged from the system as waste. Using APAM as a flocculant avoids the introduction of exogenous metal impurities.

[0057] Example 2

[0058] Based on Example 1, a seed crystal guide is added before the material is fed in step S2.

[0059] Take 10g of high-purity sodium fluoride (the amount added on a dry basis is approximately 0.75% of the theoretically generated sodium fluoride mass), grind it thoroughly until D50≈2.5μm, add 50mL of deionized water, and ultrasonically disperse for 10 minutes to obtain an aqueous seed crystal suspension. After completing step S1, add the suspension to the reaction vessel, stir for 10 minutes, and then begin adding the sodium carbonate solution. The remaining steps are the same as in Example 1.

[0060] Example 3

[0061] Based on Example 1, the feeding method in step S2 is improved.

[0062] A line-type static mixer (6 mm inner diameter, containing 12 spiral mixing units) is connected in series on the pipeline before the sodium carbonate solution enters the reactor. Simultaneously, a circulating bottom liquid (flow rate three times that of the sodium carbonate solution) is drawn from the bottom of the reactor and merges with the sodium carbonate solution at the mixer inlet, achieving high-shear premixing before entering the reactor. The remaining steps are the same as in Example 1.

[0063] Comparative Example 1

[0064] The only difference from Example 1 is that sodium polyacrylate dispersant is not added in step S1.

[0065] Results: The reaction slurry was a viscous paste, making feeding into the hydrocyclone difficult. The product purity was only 97.6%, the SiO2 content was 1.82%, and the fluorine yield was only 48.7%.

[0066] Comparative Example 2

[0067] The only difference from Example 1 is that the sodium carbonate solution is added continuously at a constant rate of 25 g / min, without stages, until the pH reaches 8.8.

[0068] Results: The product D50 was only 38μm, the fine powder rate (the mass percentage of particles with a diameter <20μm) was 18.5%, and the fluorine yield decreased to 82.3%.

[0069] Comparative Example 3

[0070] The only difference from Example 1 is that in step S3, a hydrocyclone is not used, and the mixed slurry is directly filtered by a Buchner funnel.

[0071] Results: After the third batch, the filter paper became significantly clogged, and the filtration time increased by approximately three times. The filter cloth needed to be replaced after five consecutive batches of operation, and the operation was extremely unstable.

[0072] Comparative Example 4

[0073] The only difference from Example 1 is that in step S5, 100% of the desilication mother liquor is returned for recycling, and no waste discharge is set.

[0074] Results: After continuous operation up to the 8th batch, the iron ion concentration in the mother liquor reached 87 ppm, and the purity of the product in the 10th batch dropped to 98.2%.

[0075] The test results under various process conditions are summarized in Table 1:

[0076] Table 1: Comparison of sodium fluoride product indicators and yields under various process conditions

[0077] Example 1 99.95 108 0.028 91.2 2.1 excellent Example 2 99.96 135 0.019 91.8 0.8 excellent Example 3 99.95 119 0.024 91.5 1.4 excellent Comparative Example 1 97.60 72 1.820 48.7 8.5 Difference Comparative Example 2 99.12 38 0.095 82.3 18.5 middle Comparative Example 3 99.41 103 0.052 89.1 4.2 Poor (filter media clogged) Comparative Example 4 98.20 105 0.031 92.5 3.8 Poor (accumulated impurities)

[0078] Based on the above experimental data, it can be seen that this invention, by introducing a polymeric dispersant, utilizes the steric hindrance and electrostatic repulsion provided by long-chain macromolecules to forcibly maintain the high dispersion of the byproduct silica in the mother liquor, thereby blocking the malignant co-precipitation process of silica encapsulating sodium fluoride crystals and reducing the silica content in the product (as in Comparative Example 1). Simultaneously, the viscous reaction slurry caused by silica leads to a significant reduction in fluorine yield.

[0079] This invention uses a phased crystallization guiding process, which, compared to the explosive microcrystal agglomeration caused by constant feeding (as in Comparative Example 2), can form large-particle-size regular crystals with a D50 as high as 108-135μm, which is a significant improvement over the 38μm of Comparative Example 2, and fundamentally eliminates the entrainment of impurities in the mother liquor by fine microcrystals.

[0080] In terms of economic benefits, the hydrocyclone used in this invention for centrifugal separation avoids the clogging problem of conventional filtration methods (such as Comparative Example 3), ensuring efficient separation while also guaranteeing the stability of the system's long-term operation. The waste discharge design for the mother liquor circulation system removes some residual impurities from the mother liquor, preventing the malignant accumulation of background impurities in the closed loop (as in Comparative Example 4). This ensures stable product purity while guaranteeing the overall fluoride yield, thus ensuring good economic benefits for producing sodium fluoride using this process.

[0081] Furthermore, by introducing ultrafine high-purity sodium fluoride seed crystals in advance (as in Example 2), the present invention provides a large number of highly active growth sites for the system. The introduction of seed crystals also greatly inhibits secondary nucleation and spontaneous nucleation during the reaction process, causing the supersaturation of the system to be used almost entirely for epitaxial growth attached to the seed crystal surface, ultimately producing the largest particle size and the lowest fine powder rate. Due to the further reduction of specific surface area and impurity adsorption, the purity of sodium fluoride reaches 99.96%.

[0082] The use of a pipeline static mixer and bottom liquid circulation for high-shear premixed feed avoids the localized extremely high concentration caused by directly feeding sodium carbonate solution into the reactor, achieving rapid and uniform mixing of reactants at the microscopic level. This results in a highly uniform nucleation and growth environment, effectively suppressing the generation of tiny fragments and ensuring a low fine powder rate.

[0083] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A process for preparing and controlling the morphology of high-purity sodium fluoride, characterized in that, Includes the following steps: S1. Prepare the initial base solution containing sodium fluorosilicate and a polymeric dispersant; S2. Under stirring, sodium carbonate solution is added to the initial base liquid. The pH value of the system is adjusted by controlling the feeding rate of sodium carbonate solution in stages to complete the crystallization reaction. After the pH of the system stabilizes, a mixed slurry containing sodium fluoride crystals and dispersed silica is obtained. S3. The mixed slurry is introduced into a hydrocyclone for classification and separation. The crude sodium fluoride is collected by the underflow and the mother liquor containing silica is obtained by the overflow. S4. The crude sodium fluoride product is purified to obtain high-purity sodium fluoride product. S5. The overflowing silica-containing mother liquor is subjected to desiliconization solid-liquid separation to obtain desiliconized mother liquor. The desiliconized mother liquor is divided into a circulating stream and a waste stream in proportion. The circulating stream is returned to step S1 or step S2 for recycling.

2. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that, In step S1, the polymeric dispersant is selected from at least one of polyethylene glycol, polyacrylamide, sodium polyacrylate, and polyvinyl alcohol; The amount of the polymeric dispersant added is 0.01% to 1.0% of the mass of sodium fluorosilicate.

3. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that, Before performing step S2 and adding sodium carbonate solution, the process also includes a step of pre-adding ultrafine high-purity sodium fluoride seeds to the initial base solution; The average particle size D50 of the sodium fluoride seed crystals is controlled between 1 and 5 μm, and the sodium fluoride seed crystals are added to the system in the form of a pre-dispersed aqueous suspension. The amount of seed crystals added is 0.5% to 1.0% of the theoretical mass of sodium fluoride produced.

4. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that, In step S2, the sodium carbonate solution is injected into the reaction system through a pipeline static mixer or a high-shear emulsifier, so that the added sodium carbonate solution and the initial base liquid achieve high-shear dispersion and mixing at the moment of injection.

5. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that, In step S2, the crystallization reaction temperature is controlled at 85–90°C; the step-by-step control of the sodium carbonate solution feeding rate to achieve gradient adjustment of the system's pH value specifically includes: Rapid nucleation stage: Sodium carbonate solution is added to the system at the first feeding rate. During this stage, the pH value of the system rises from the initial value to 6.5-7.

5. Crystal growth stage: Reduce the stirring speed of the system and continue to add sodium carbonate solution at a second feeding rate that is less than the first feeding rate until the pH value of the system reaches the reaction endpoint of 8.5 to 9.

0.

6. The preparation and morphology control process of high-purity sodium fluoride according to claim 5, characterized in that, The second feeding rate is set to 10% to 20% of the first feeding rate.

7. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that: In step S4, the purification process includes washing and drying; wherein the washing is performed using pure water in a countercurrent manner; and the drying is performed using a fluidized bed drying system under hot air conditions at 100-120°C.

8. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that: In step S5, the specific operation of the desiliconization solid-liquid separation is as follows: a flocculant is added to the silica-containing mother liquor to cause the suspended silica to coagulate and settle, and the desiliconized mother liquor is obtained after pressure filtration separation; The flocculant is a metal ion-free organic polymer flocculant.

9. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that: In step S5, 80% to 85% of the total amount of the desilication mother liquor is returned to the recycling system, and the remaining 15% to 20% is discharged from the waste system.

10. The preparation and morphology control process of high-purity sodium fluoride according to claim 1, characterized in that: Throughout the entire process system, all reaction devices, agitators, hydrocyclones, and conveying pipelines that come into contact with fluorine-containing materials have polytetrafluoroethylene or perfluoroalkoxy resin as their inner surface material or coating.