Preparation method of cryolite with low silicon and high molecular ratio and product thereof

Abstract

The invention provides a preparation method of cryolite with low silicon and high molecular ratio, which comprises the following steps: preparing sodium difluoride by taking hydrofluoric acid and sodium fluoride as raw materials, and carrying out complexation reaction on the sodium difluoride and sodium aluminate to improve the content of sodium separated out by crystallization so as to improve the Na / Al molecular ratio of the cryolite; hydrofluoric acid and sodium fluoride are used as raw materials, and generation of silicon dioxide in a reaction system can be avoided, so that the content of impurity silicon dioxide in high-molecular-ratio cryolite is reduced; in addition, hydrofluoric acid as a byproduct in the lithium hexafluorophosphate and fluoroethylene carbonate industry can be effectively utilized, so that the raw material cost is reduced.

Description

Technical Field

[0001] The invention belongs to the field of inorganic fluoride preparation, and in particular relates to a preparation method of low-silicon high-molecular-weight cryolite and its product. Background Art

[0002] Cryolite, also known as sodium fluoroaluminate, has the molecular formula Na₃AlF₆. Due to impurities in cryolite, its Na / Al molecular ratio is not the theoretical value of 3:1. Based on the Na / Al ratio, cryolite can be divided into high-molecular-weight cryolite (Na / Al molecular ratio: 2.8-3.0) and low-molecular-weight cryolite (Na / Al molecular ratio: less than 2.8). High-molecular-weight cryolite has the advantages of low volatility, good thermal stability, and low environmental pollution. It is widely used as a flux in aluminum electrolysis and is widely used in aluminum smelters. Silicon dioxide is a key impurity in high-molecular-weight cryolite. During the aluminum electrolysis process, silicon dioxide not only disrupts the electrolyte balance and reduces conductivity, but also accelerates corrosion in the electrolytic cell, shortening the cell life. Therefore, the silicon dioxide content in high-molecular-weight cryolite must be controlled below 0.25%.

[0003] Patent CN105645447A discloses a method for producing high-molecular-weight cryolite using low-concentration fluorosilicic acid. The method involves preparing an aluminum fluoride solution using fluorosilicic acid and aluminum hydroxide. The aluminum fluoride solution is then mixed with sodium fluoride at a molar ratio of 1:(2.8-3.2) and stirred for reaction. The mixture is then filtered and flash-dried to produce high-molecular-weight cryolite. Patent CN1225897A discloses a method for producing high-molecular-weight cryolite. The method involves using sodium fluorosilicate and sodium aluminate as raw materials. At room temperature and pressure, sodium fluorosilicate is decomposed with industrial ammonia to produce a fluorine-containing solution. The fluorine-containing solution and sodium aluminate solution react at 90-100°C to produce a cryolite slurry. The slurry is then filtered and dried to produce high-molecular-weight cryolite. The above methods all use fluorosilicic acid or sodium fluorosilicate as raw materials. However, fluorosilicic acid or sodium fluorosilicate contains a large amount of silicon dioxide, which is difficult to separate during the preparation process, resulting in a higher silicon dioxide content in the polymer cryolite product. At the same time, a large amount of fluorine-containing silicon slag is also produced as a by-product, seriously polluting the environment. Summary of the Invention

[0004] In order to solve the above technical problems, the first aspect of the present invention provides a method for preparing low-silicon high-molecular-weight cryolite, comprising the following steps: Step S1, preparing sodium aluminate: adding aluminum hydroxide to a sodium hydroxide aqueous solution, stirring and dissolving, to obtain a sodium aluminate solution; Step S2, preparing sodium difluoride acid solution: adding sodium fluoride to hydrofluoric acid, stirring and dissolving, to obtain sodium difluoride acid solution; Step S3, preparation of high molecular weight cryolite: adding the sodium aluminate solution obtained in step S1 and the sodium bifluoride acid solution obtained in step S2 into a reactor to carry out a crystallization reaction to obtain cryolite slurry; Step S4, filtering and drying: The cryolite slurry obtained in step S3 is filtered, centrifuged, and dried to obtain high molecular weight cryolite.

[0005] The reaction principle of the above technical solution is as follows, and the process flow chart is shown in Figure 1 .

[0006]

[0007] As a preferred embodiment, the mass concentration of the sodium hydroxide aqueous solution in step S1 is 12% to 18%.

[0008] As a preferred embodiment, in the sodium aluminate preparation process in step S1, the molar ratio of sodium hydroxide to aluminum hydroxide is 1.8 to 2.0.

[0009] As a preferred solution, the dissolution temperature during the preparation of sodium aluminate in step S1 is 60°C to 80°C.

[0010] As a preferred embodiment, the hydrofluoric acid in step S2 is derived from a by-product of lithium hexafluorophosphate or fluoroethylene carbonate.

[0011] As a preferred embodiment, the mass concentration of hydrofluoric acid in step S2 is 10% to 20%.

[0012] As a preferred embodiment, the molar ratio of sodium fluoride to hydrofluoric acid in step S2 is 0.16-0.25.

[0013] As a preferred embodiment, the mass content of impurity silicon dioxide in the sodium fluoride in step S2 is ≤0.5%.

[0014] As a preferred solution, the sodium aluminate solution and the sodium bifluoride acid solution are added simultaneously in step S3.

[0015] As a preferred embodiment, the pH value of the crystallization reaction in step S3 is 8-10.

[0016] As a preferred embodiment, the crystallization reaction temperature in step S3 is 50°C to 70°C.

[0017] As a preferred solution, the drying temperature in step S4 is 110° C. to 130° C., and the drying time is 1 hour to 3 hours.

[0018] As a preferred solution, in step S4, the Na / Al molecular ratio of the high molecular weight cryolite is greater than 2.8, and the mass content of silicon dioxide is less than 0.25%.

[0019] The second aspect of the present invention provides a low-silicon high-molecular-weight cryolite, which is prepared by the above-mentioned preparation method.

[0020] Through the above technical solution, the present invention produces the following beneficial effects: (1) The present invention uses hydrofluoric acid and sodium fluoride as raw materials to prepare sodium difluoride. Sodium difluoride is complexed with sodium aluminate. When aluminate ions are complexed with fluoride ions, they are also combined with sodium ions, thereby increasing the sodium content of crystallized precipitation and improving the Na / Al molecular ratio of cryolite to above 2.8, meeting the quality requirements of high molecular ratio.

[0021] (2) When fluorosilicic acid and sodium fluorosilicate are used as raw materials, a large amount of silicon dioxide is produced in the reaction system. The present invention uses hydrofluoric acid and sodium fluoride as raw materials instead of fluorosilicic acid and sodium fluorosilicate, thereby avoiding the production of silicon dioxide in the reaction system and effectively reducing the silicon dioxide content in the system, thereby reducing the silicon dioxide content in the high molecular weight cryolite product and stably controlling it to below 0.25%.

[0022] (3) The present invention effectively utilizes lithium hexafluorophosphate and hydrofluoric acid produced as a by-product in the fluoroethylene carbonate industry, thereby reducing the cost of raw materials, ensuring the market competitiveness of the product, and providing a technically and economically feasible solution for the high molecular weight cryolite industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the production process flow chart of low silicon high molecular weight cryolite. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to the examples.

[0025] Example 1 Step S1, preparation of sodium aluminate: dissolve 100 g of sodium hydroxide in water to prepare an alkaline solution with a mass concentration of 15%, then add 108 g of aluminum hydroxide solid to the alkaline solution, heat and stir until the solution is clear, heating the temperature to 60°C to 70°C, to obtain a sodium aluminate solution; Step S2, preparing sodium difluoride acid solution: adding 42 g of low-silicon sodium fluoride to 500 g of 10% by-product hydrofluoric acid, stirring and dissolving at room temperature until the solution is clear, to obtain sodium difluoride acid solution; Step S3, preparation of high molecular weight cryolite: the sodium aluminate solution obtained in step S1 and the sodium bifluoride acid solution obtained in step S2 are added to a reactor simultaneously, heated and stirred to perform a crystallization reaction, the reaction temperature is controlled between 50° C. and 60° C., and the pH value is controlled between 8 and 9 during the reaction process, and cryolite slurry is obtained after the reaction; Step S4, filtering and drying: The cryolite slurry obtained in step S3 is filtered, centrifuged, and dried at 120° C. for 2 h to obtain high molecular weight cryolite.

[0026] Example 2 Step S1, preparation of sodium aluminate: dissolve 100 g of sodium hydroxide in water to prepare an alkaline solution with a mass concentration of 15%, then add 102 g of aluminum hydroxide solid to the alkaline solution, heat and stir until the solution is clear, heating the temperature to 60°C to 70°C, to obtain a sodium aluminate solution; Step S2, preparing sodium difluoride acid solution: adding 58 g of low-silicon sodium fluoride to 355 g of 15% by-product hydrofluoric acid, stirring and dissolving at room temperature until the solution is clear, to obtain sodium difluoride acid solution; Step S3, preparation of high molecular weight cryolite: the sodium aluminate solution obtained in step S1 and the sodium bifluoride acid solution obtained in step S2 are added to a reactor simultaneously, heated and stirred to perform a crystallization reaction, the reaction temperature is controlled between 50° C. and 60° C., and the pH value is controlled between 8 and 9 during the reaction process, and cryolite slurry is obtained after the reaction; Step S4, filtering and drying: The cryolite slurry obtained in step S3 is filtered, centrifuged, and dried at 120° C. for 2 h to obtain high molecular weight cryolite.

[0027] Example 3 Step S1, preparation of sodium aluminate: dissolve 100 g of sodium hydroxide in water to prepare an alkaline solution with a mass concentration of 15%, then add 102 g of aluminum hydroxide solid to the alkaline solution, heat and stir until the solution is clear, heating temperature is 70°C to 80°C, to obtain a sodium aluminate solution; Step S2, preparing sodium difluoride acid solution: adding 52.5 g of sodium fluoride to 333 g of 15% by-product hydrofluoric acid, stirring and dissolving at room temperature until the solution is clear, to obtain sodium difluoride acid solution; Step S3, preparation of high molecular weight cryolite: the sodium aluminate solution obtained in step S1 and the sodium bifluoride acid solution obtained in step S2 are added to a reactor simultaneously, heated and stirred to perform a crystallization reaction, the reaction temperature is controlled between 60° C. and 70° C., and the pH value is controlled between 9 and 10 during the reaction process, and cryolite slurry is obtained after the reaction; Step S4, filtering and drying: The cryolite slurry obtained in step S3 is filtered, centrifuged, and dried at 120° C. for 2 h to obtain high molecular weight cryolite.

[0028] Example 4 Step S1, preparation of sodium aluminate: dissolve 100 g of sodium hydroxide in water to prepare an alkaline solution with a mass concentration of 15%, then add 97 g of aluminum hydroxide solid to the alkaline solution, heat and stir until the solution is clear, heating the temperature to 70°C to 80°C, to obtain a sodium aluminate solution; Step S2, preparing sodium difluoride acid solution: adding 70 g of low-silicon sodium fluoride to 267 g of 20% by-product hydrofluoric acid, stirring and dissolving at room temperature until the solution is clear, to obtain sodium difluoride acid solution; Step S3, preparation of high molecular weight cryolite: the sodium aluminate solution obtained in step S1 and the sodium bifluoride acid solution obtained in step S2 are added to a reactor simultaneously, heated and stirred to perform a crystallization reaction, the reaction temperature is controlled between 60° C. and 70° C., and the pH value is controlled between 9 and 10 during the reaction process, and cryolite slurry is obtained after the reaction; Step S4, filtering and drying: The cryolite slurry obtained in step S3 is filtered, centrifuged, and dried at 120° C. for 2 h to obtain high molecular weight cryolite.

[0029] The quality test results of the low-silicon, high-molecular-weight cryolite products obtained in the examples are shown in Table 1. These results demonstrate that the technical solution of the present invention can effectively reduce the impurity silica content in cryolite and increase the Na / Al ratio. The product quality meets the quality requirements of high-molecular-weight cryolite CH-0 as specified in GB / T 4291-2017. The quality consistency between different examples is high, and the production process is stable and reliable.

[0030] Table 1 Analysis results of low silicon high molecular weight cryolite products produced by the present invention

[0031] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing low-silicon high-molecular-weight cryolite, characterized in that: The following steps are involved: Step S1, preparing sodium aluminate: adding aluminum hydroxide to a sodium hydroxide aqueous solution, stirring and dissolving, to obtain a sodium aluminate solution; Step S2, preparing sodium difluoride acid solution: adding sodium fluoride to hydrofluoric acid, stirring and dissolving, to obtain sodium difluoride acid solution; Step S3, preparation of high molecular weight cryolite: adding the sodium aluminate solution obtained in step S1 and the sodium bifluoride acid solution obtained in step S2 into a reactor to carry out a crystallization reaction to obtain cryolite slurry; Step S4, filtering and drying: The cryolite slurry obtained in step S3 is filtered, centrifuged, and dried to obtain cryolite with a low silicon and high molecular weight ratio.

2. The preparation method according to claim 1, characterized in that The mass concentration of the sodium hydroxide aqueous solution in step S1 is 12% to 18%.

3. The preparation method according to claim 2, characterized in that In the sodium aluminate preparation process in step S1, the molar ratio of sodium hydroxide to aluminum hydroxide is 1.8-2.

0.

4. The preparation method according to claim 3, characterized in that The dissolution temperature during the preparation of sodium aluminate in step S1 is 60° C. to 80° C.

5. The preparation method according to claim 4, characterized in that The hydrofluoric acid in step S2 is derived from a by-product of lithium hexafluorophosphate or fluoroethylene carbonate.

6. The preparation method according to claim 5, characterized in that The mass concentration of hydrofluoric acid in step S2 is 10% to 20%.

7. The preparation method according to claim 6, characterized in that In step S2, the molar ratio of sodium fluoride to hydrofluoric acid is 0.16-0.

25.

8. The preparation method according to claim 7, characterized in that The mass content of impurity silicon dioxide in the sodium fluoride in step S2 is ≤0.5%.

9. The preparation method according to claim 8, characterized in that In step S3, the sodium aluminate solution and the sodium bifluoride acid solution are added simultaneously.

10. A low silicon high molecular weight cryolite, characterized in that: The low-silicon high-molecular-weight cryolite is prepared by the preparation method according to claim 1.

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