A method for producing cryolite

Abstract

This invention belongs to the field of inorganic fluorine chemical technology, specifically relating to a method for preparing cryolite. Sodium fluoride and sodium aluminate are dispersed in water to obtain a suspension, which is heated to 60-80°C. Carbon dioxide is introduced under an electric field while the mixture is stirred for 2-4 hours. The mixture then settles, and the solid and liquid are separated. The solid is washed and dried to obtain the cryolite product. This invention introduces a direct current electric field as a physical field control method into the traditional wet precipitation process. Without introducing any external organic or inorganic additives, it achieves precise control over the crystal nucleation rate and growth direction, ultimately obtaining cryolite with a uniform octahedral structure. This method features mild process conditions, low equipment cost, and is suitable for industrialization. The resulting product has both good fluidity and high purity, fully meeting the technical requirements of modern aluminum electrolysis industry for high-quality cryolite flux, and has significant economic value and environmental benefits.

Description

Technical Field

[0001] This invention belongs to the field of inorganic fluorine chemical technology, specifically relating to a method for preparing cryolite. Background Technology

[0002] Cryolite (Na3AlF6), an indispensable flux in the aluminum electrolysis industry, significantly impacts the stability, energy consumption, and operational efficiency of the electrolysis process due to its physical morphology, particle size distribution, and flowability. Traditional wet precipitation methods for cryolite preparation typically rely on the reaction of sodium fluorosilicate, hydrofluoric acid, or fluorine-containing waste liquid with an aluminum source in an aqueous phase, achieving crystallization by adjusting pH, temperature, and stirring conditions. While this method is mature and uses widely available raw materials, it lacks effective means to control crystal orientation and particle morphology during crystal growth. The resulting product has a small particle size, is often irregularly shaped, easily absorbs moisture, and has poor flowability. This can lead to uneven distribution during electrolytic cell feeding and may exacerbate fluoride volatilization and dust dispersion, affecting the working environment and electrolysis efficiency.

[0003] In existing technologies, some patents attempt to improve the physical properties of cryolite by optimizing the reaction system or introducing specific additives. For example, patent CN109796034B (Institute of Process Engineering, Chinese Academy of Sciences) proposes a method for recovering and preparing high-quality cryolite from silicon- and fluorine-containing wastewater. Through steps such as ammonia desilication and sodium aluminate synthesis, a cryolite product with high purity, SiO2 content below 0.1%, and good fluidity is obtained. This method has advantages in resource recycling and environmental protection, but its crystallization process still relies on conventional stirring and natural sedimentation, without actively intervening in crystal nucleation and growth direction. The resulting particle morphology is still randomly controlled by thermodynamic and kinetic factors such as solution supersaturation and ion diffusion rate, making it difficult to achieve preferred crystal orientation or directional construction of regular spherical structures.

[0004] Another representative technology is patent CN119100427A (Huzhou University), which involves co-dissolving inorganic aluminum salts and organic acid sodium salts, introducing fluorinating agents, and then preparing cryolite microspheres via a hydrothermal reaction. It claims to be able to control the size and wall thickness of the microspheres. While this method has explored morphology control, it relies on organic acid sodium salts as structure-directing agents, posing a risk of introducing organic impurities. If such organic components remain in the final product, they may decompose in the high-temperature electrolysis environment, producing carbonaceous or volatile byproducts, interfering with the electrolyte composition, and even affecting the purity of the aluminum melt, thus limiting its application in high-end electrolytic aluminum processes.

[0005] In summary, while existing cryolite preparation technologies achieve high purity, good flowability, and regular morphology, they still lack a green synthesis strategy that does not introduce foreign impurities and allows for precise control of crystal growth under normal pressure and mild conditions. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this application provides a method for preparing cryolite by introducing a DC electric field into a wet precipitation reaction system to regulate the migration behavior of fluorine-aluminum complex anions and the crystal growth orientation, thereby obtaining smooth-surfaced and regularly shaped octahedral cryolite particles. This method not only improves the bulk density and flowability of the product, but also avoids the use of organic additives or template agents, thus preventing electrolyte contamination.

[0007] Specifically, a method for preparing cryolite includes the following steps: dispersing sodium fluoride and sodium aluminate in water to obtain a suspension, heating to 60-80℃, introducing carbon dioxide under an electric field and stirring for 2-4 hours, then separating the solid and liquid, washing and drying the solid to obtain the cryolite product. The cryolite prepared by the above method has a D50 particle size of 25-50 μm, a bulk density of 1.15-1.35 g / cm³, and a particle size distribution span (Span value) of 0.8-1.2, exhibiting a concentrated particle size distribution that facilitates uniform dissolution in an electrolytic cell.

[0008] Furthermore, the amounts of sodium fluoride and sodium aluminate satisfy the F:Al molar ratio of 6.1-6.6:1, the solid-liquid ratio of the suspension is 10-30:100, and the amount of carbon dioxide used is 5-6 times the number of moles of sodium aluminate.

[0009] Furthermore, the electric field strength is 50-200V / m, the electrodes are arranged in parallel, the electrode spacing is 10-30cm, and the electrode material used is a titanium-based composite electrode coated with a platinum layer.

[0010] Furthermore, the electric field strength changes from 200 to 50 V / m within 10 seconds, and repeats the change with a period of 10 seconds.

[0011] Preferably, the stirring speed is 150-300 rpm, and after the reaction is completed, the electric field is turned off 0-5 minutes earlier than the stirring stop time.

[0012] Preferably, the solid drying temperature is 100-120℃ and the time is 6-16h.

[0013] In some embodiments, the sodium fluoride is prepared by reacting sodium fluorosilicate with sodium carbonate solution at 80-100°C.

[0014] In some implementations, after stirring is stopped, the mixture is allowed to stand for 10-30 minutes to allow the solid particles to settle, and then the solid and liquid are separated.

[0015] In some embodiments, the reaction vessel used for the reaction is made of Hastelloy or titanium with a polytetrafluoroethylene lining, and the reaction vessel integrates a temperature sensor. The agitator used is a three-layer paddle structure, with the upper layer being a propeller-type paddle, the middle layer being a straight-bladed turbine, and the lower layer being an anchor-type paddle.

[0016] In some embodiments, the reactor is provided with a constant temperature jacket, and the circulating medium inside the jacket is heat transfer oil or deionized water, which is used to maintain the temperature fluctuation of the reaction system not exceeding ±2℃.

[0017] In some embodiments, the electric field direction is perpendicular to the direction of gravity, and the electrode plane is kept parallel to the axis of rotation of the agitator to reduce the interference of fluid shear on the electric field distribution.

[0018] It is understandable that during the reaction of sodium fluoride and sodium aluminate to form cryolite, applying a DC electric field of a specific intensity range in the reaction system causes the negatively charged AlF6 to... 3- Under the influence of the electric field, the complexed anions migrate directionally along the direction of the electric field, suppressing the rapid nucleation phenomenon caused by excessive supersaturation in the local area. At the same time, the electric field induces the crystal to preferentially grow along the low surface energy {111} crystal plane. At this time, the growth rate of the crystal is greater than the formation rate of the crystal nucleus, which promotes the crystal nucleus to eventually develop into a regular octahedral structure with a smooth surface and regular geometric shape.

[0019] In some embodiments, the resulting cryolite product has a Na / Al atomic ratio of 2.95-3.05, an F / Al atomic ratio of 5.90-6.10, a SiO2 content of less than 0.05%wt, and a Fe2O3 content of less than 0.01%wt.

[0020] In some implementations, the angle of repose of the resulting cryolite product is less than 35°, indicating good fluidity and suitability for pneumatic conveying or automatic feeding systems.

[0021] Compared with the prior art, the present invention has the following beneficial effects: This invention introduces a direct current electric field as a physical field control method into the traditional wet precipitation process. Without introducing any external organic or inorganic additives, it achieves precise control over the crystal nucleation rate and growth direction, ultimately yielding cryolite with a uniform octahedral structure. This method features mild process conditions, low equipment costs, and is suitable for industrialization. The resulting product possesses both good flowability and high purity, fully meeting the technical requirements of modern aluminum electrolysis for high-quality cryolite flux, and exhibits significant economic value and environmental benefits. Attached Figure Description

[0022] Figure 1 This is a comparison diagram of the XRD pattern of the cryolite product obtained in Example 1 and the cryolite PDF standard card.

[0023] Figure 2 The image shows the SEM image of the cryolite product obtained in Example 1.

[0024] Figure 3 The image shows the SEM image of the cryolite product obtained in Comparative Example 1. Detailed Implementation

[0025] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. These embodiments are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention or the method of implementing the present invention.

[0026] The reaction equation for the method of preparing cryolite in this invention is as follows: 6NaF + NaAlO2+ 4CO2+ 2H2O → Na3AlF6↓+ 4NaHCO3 The byproducts may be NaHCO3, Na2CO3, or a mixture of the two, depending on the amount of CO2 used. A larger amount of CO2 will produce NaHCO3, while a smaller amount will produce Na2CO3.

[0027] Example 1 Preparation of highly dispersible cryolite.

[0028] In a stirred tank equipped with parallel electrodes, 265g of sodium fluoride and 82g of sodium aluminate (molar ratio 6.3:1) were dispersed in 1200g of water to obtain a suspension. The temperature was raised to 70℃, and stirring was started. The electrodes were connected to a 15V DC power supply with an electrode spacing of 10cm (electric field strength 150V / m). Carbon dioxide was introduced at a rate of 0.7L / min and stirred continuously at 300rpm for 3 hours. A total of 240g of carbon dioxide was used. The power supply was turned off and stirring was stopped. The mixture was then allowed to settle for 30 minutes. After filtration, the solid was washed with pure water until the wash water was neutral. The solid was then dried in an oven at 105℃ for 12 hours to obtain 188.7g of white powdered cryolite product.

[0029] Example 2 Preparation of highly dispersible cryolite.

[0030] In a stirred tank equipped with parallel electrodes, 257g of sodium fluoride and 82g of sodium aluminate (molar ratio 6.1:1) were dispersed in 2000g of water to obtain a suspension. The temperature was raised to 60℃, and stirring was started. The electrodes were connected to a 10V DC power supply with an electrode spacing of 10cm (electric field strength 100V / m). Carbon dioxide was introduced at a rate of 0.6L / min and stirred continuously at 250rpm for 4 hours. A total of 264g of carbon dioxide was used. The power was turned off for 5 minutes and stirring was stopped. The mixture was then allowed to settle for 20 minutes. After filtration, the solid was washed with pure water until the wash water was neutral. The solid was then dried in a 120℃ oven for 6 hours to obtain 185g of white powdered cryolite product.

[0031] Example 3 Preparation of highly dispersible cryolite.

[0032] In a stirred tank equipped with parallel electrodes, 277g of sodium fluoride and 82g of sodium aluminate (molar ratio 6.6:1) were dispersed in 1600g of water to obtain a suspension. The temperature was raised to 80℃, and stirring was started. The electrodes were connected to a 12V DC power supply with an electrode spacing of 15cm (electric field strength 80V / m). Carbon dioxide was introduced at a rate of 1.0L / min and stirred continuously at 200rpm for 2 hours. A total of 224g of carbon dioxide was used. The power supply was turned off and stirring was stopped. The mixture was then allowed to settle for 20 minutes. After filtration, the solid was washed with pure water until the wash water was neutral. The solid was then dried in an oven at 110℃ for 8 hours to obtain 192.3g of white powdered cryolite product.

[0033] Example 4 Preparation of highly dispersible cryolite.

[0034] In a stirred tank equipped with parallel electrodes, 269g of sodium fluoride and 82g of sodium aluminate (molar ratio 6.4:1) were dispersed in 1500g of water to obtain a suspension. The temperature was raised to 75℃, and stirring was started. The electrodes were connected to a changing DC power supply, and the power supply voltage changed uniformly from 20V to 5V within 10 seconds, and this change was repeated continuously. The electrode spacing was 10cm (the electric field strength changed uniformly from 200V / m to 50V / m). Carbon dioxide was introduced at a rate of 0.7L / min and stirring was carried out continuously at 300rpm. The reaction was carried out for 3h, and a total of 240g of carbon dioxide was used. The power was turned off for 5min and stirring was stopped. Then, the mixture was allowed to settle for 30min. After filtration, the solid was washed with pure water until the wash water was neutral. It was then dried in an oven at 105℃ for 12h to obtain 189g of white powdered cryolite product.

[0035] Example 5 Prepare raw material sodium fluoride 1 kg of sodium fluorosilicate was dispersed in 3 kg of water and then mixed with 13 kg of saturated sodium carbonate solution. The mixture was reacted at 90 °C for 5 h. After filtering out silica gel, the solution was evaporated, concentrated, and cooled to crystallize, yielding 1.1 kg of sodium fluoride.

[0036] Comparative Example 1 The preparation of cryolite was basically the same as in Example 1, except that the electrodes were not connected to a power source. 187g of white powdered cryolite product was obtained.

[0037] The cryolite products obtained in the above examples and comparative examples were subjected to performance tests. Particle size, bulk density, angle of repose, etc., were tested according to parts 13-14 of standards GB / T 19077-2016 and YS / T 581-2006. The results are shown in the table below: As shown in the table above, the cryolite products obtained by applying an electric field in Examples 1-4 are superior to the comparative products without an applied electric field in terms of particle size and flowability.

Claims

1. A method for the production of cryolite, characterized in that, The method comprises the following steps: The sodium fluoride and sodium aluminate are dispersed in water to form a suspension, the temperature is raised to 60-80 DEG C, carbon dioxide is bubbled in under the action of an electric field and the reaction is stirred for 2-4 h, then solid-liquid separation is performed, the solid is washed and dried to obtain the cryolite product.

2. The method of claim 1, wherein, The amount of sodium fluoride and sodium aluminate is such that the F:Al molar ratio is 6.1-6.6:1, and the solid-liquid ratio of the suspension is 10-30:

100.

3. The method of claim 1, wherein, The amount of carbon dioxide is 5-6 times the molar amount of sodium aluminate.

4. The method of claim 1, wherein, The electric field strength is 50-200 V / m, the electrode spacing is 10-30 cm, and the electrode material is a titanium-based composite electrode coated with a platinum layer.

5. The method of claim 4, wherein, The electric field strength is changed from 200 to 50 V / m in 10 s, and the change is repeated in cycles of 10 s.

6. The method of claim 1, wherein, The stirring speed is 150-300 rpm, and the electric field is turned off 0-5 min earlier than the stirring is stopped.

7. The method of claim 1, wherein, The drying temperature is 100-120 DEG C, and the drying time is 6-16 h.

8. The method of claim 1, wherein, The sodium fluoride is prepared by reacting sodium fluosilicate with a sodium carbonate solution at 80-100 DEG C.

9. The method of claim 1, wherein, The material of the reaction kettle is Hastelloy or titanium with a polytetrafluoroethylene lining, and the stirrer is a three-layer paddle structure, with a propeller blade in the upper layer, a flat blade turbine in the middle layer, and an anchor paddle in the lower layer.

Images