Method for preparing cryolite by using rare earth molten salt electrolytic slag alkaline leaching solution

Abstract

The invention provides a method for preparing cryolite by using rare earth molten salt electrolytic slag alkaline leaching liquid. The method comprises the following steps: firstly, decomposing rare earth molten salt slag through a sub-molten salt technology, separating rare earth and fluorine in the rare earth molten salt slag through water washing to obtain a fluorine-containing alkaline solution, and then enriching fluorine in the solution through simple evaporation and concentration to obtain a high-alkali fluorine-containing solution. The fluorine-containing solution is added into an aluminum sulfate solution, aluminum ions necessary for cryolite synthesis are introduced, the pH value, the composition proportion, the temperature and the reaction time are adjusted, high-molecular-ratio cryolite is finally prepared, and resource utilization of fluorine in the molten salt slag is achieved. According to the method, fluorine in the slag can be efficiently recycled while rare earth is efficiently recycled from the rare earth molten salt electrolysis slag, the recycling rate of fluorine is larger than 95%, the process is simple, only simple impurity removal is needed, gradient separation of various ions is not needed, acid waste gas is not prone to being generated under the alkaline condition, and safety is higher. The prepared cryolite meets national standard requirements, and the process is simple and easy to operate.

Description

Technical Field

[0001] The invention relates to the technical field of resource recycling, and in particular to a method for preparing cryolite by utilizing rare earth molten salt electrolytic slag alkali leaching solution. Background Art

[0002] Rare earth molten salt electrolytic slag is a rare earth secondary resource produced during the electrolysis of rare earth molten salt, which contains about 10-80% rare earth oxides. Rare earths mainly exist in the form of rare earth fluoride and rare earth oxyfluoride. Every time molten salt electrolysis produces rare earth metals, about 5% of rare earths are lost to the bottom of the electrolytic cell and become rare earth molten salt slag. According to the current annual output of rare earth smelting in my country of about 100,000 tons, about 5,000 tons of rare earth oxides are lost each year. At present, the process of treating rare earth molten salt slag mainly adopts sulfuric acid aging and salt roasting. The fluorine in the sulfuric acid method will turn into hydrofluoric acid gas overflow and is difficult to recover, while the salt roasting will produce calcium fluoride solid with more impurities, which will not only carry rare earths, but also be difficult to remove impurities, making it difficult to use as a resource product. In general, the process of treating rare earth molten salt slag is more focused on the recycling of rare earth resources, and more on removing fluorine as an impurity, and there is less research on the recovery of fluorine resources.

[0003] As an important strategic resource, fluorine is a strategic reserve that the country attaches great importance to. Thanks to its excellent physical and chemical properties, fluorine chemical products are widely used, and the main application fields include aviation, military industry, precision instruments, electronic equipment, etc. Nearly 90% of the fluorine resources currently used come from fluorite, and the remaining 10% comes from sodium fluorosilicate (a by-product of phosphate fertilizer). According to relevant statistical data, China's fluorine resource reserves account for 54% of the world's total, and 60% of the global fluorine resource supply comes from China, which is much higher than the resource share. Fluorine-containing wastewater is an important secondary resource. Ignoring it will not only cause serious pollution, but also be a huge waste of resources. Therefore, for the sake of resource protection and sustainable development of the industry, it is of long-term significance to recycle fluorine in wastewater as a resource.

[0004] Sub-molten salt medium is a high-concentration alkaline medium with excellent physical and chemical properties and can efficiently decompose metal minerals. Regarding the previous patent CN114774718B, when sub-molten salt technology is used to treat rare earth molten salt slag, the wastewater generated by water washing and defluorination is characterized by high sodium, fluorine and alkalinity. It is an important fluorine-containing secondary resource. How to utilize it as a resource is not only a supplement to the fluorine-containing wastewater resource technology, but also an important part of the full recovery of valuable metals in rare earth molten salt electrolytic slag. How to synthesize qualified fluorine resource products from high-alkaline fluorine-containing solutions and achieve efficient recovery of most of the valuable elements therein are problems that need to be solved in the comprehensive utilization technology of fluorine-containing wastewater. Solving this problem has important environmental and economic value.

[0005] Cryolite is a fluorine-aluminum complex salt with many excellent physical and chemical properties. It is widely used in aluminum electrolysis industry, agricultural pesticides, etc. It has a high price and has economic value as a resource product of fluorine. In addition, the solubility of cryolite is extremely low. If it is recycled as a resource product, the fluorine recovery rate in alkaline fluorine-containing waste liquid must be higher than other resource product recovery methods. Therefore, the use of fluorine in molten salt slag to prepare synthetic cryolite has both environmental and economic benefits.

[0006] The prior art discloses a method for resource utilization of fluorine in rare earth molten salt electrolytic slag: the rare earth molten salt electrolytic slag is subjected to aluminum sulfate and calcium oxide co-roasting, acid leaching to obtain a solution containing rare earth and fluorine, and then rare earth is separated and extracted through a rare earth complex salt precipitation process, and filtered to obtain a fluorine-containing solution. The fluorine-containing solution is purified by iron removal, pH value and component ratio adjustment to finally prepare high molecular ratio cryolite; the above scheme has the following disadvantages: a. High energy consumption: the cryolite precipitation process is lengthy and consumes various reagents; b. Regardless of the loss rate of rare earth or fluorine and aluminum, the precipitation of rare earth complex salts will entrain fluorine and aluminum ions, and the pH adjustment process of iron precipitation will also entrain some ions; c. It is carried out under acidic conditions, and acidic waste gas will be generated during heating.

[0007] In summary, the current resource utilization of fluorine in rare earth molten salt slag has the following defects: 1. Rare earths and various ions such as fluorine and aluminum exist simultaneously in the liquid phase, resulting in a lengthy resource separation process and easy loss due to entrainment; 2. In the relevant resource recovery process under an acidic environment, heating is likely to produce acidic waste gas.

[0008] Based on the defects in the current resource utilization of fluorine in rare earth molten salt slag, it is necessary to improve it. Summary of the invention

[0009] The purpose of the present invention is to provide a method for preparing cryolite by utilizing alkaline leaching of rare earth molten salt electrolytic slag. The advantages of the method are: first, the process is simple (only simple evaporation and concentration are required, and basically no impurity removal step is required); second, it is environmentally friendly and economical (synthesized in an alkaline environment, and no acidic waste gas is generated); first, the rare earth molten salt electrolytic slag is treated with sub-molten salt, and then washed with water to obtain an alkaline fluorine-containing solution, and then aluminum ions necessary for synthesizing cryolite are introduced into the alkaline solution, and then the composition ratio is adjusted, the pH, temperature and reaction time are adjusted, and finally a qualified cryolite product is prepared, thereby realizing a supplement to the resource utilization technology of fluorine-containing wastewater and the resource utilization of fluorine in molten salt slag.

[0010] To achieve the above object, the present invention provides the following technical solutions:

[0011] The present invention provides a method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution, comprising the following steps:

[0012] Adding a sub-molten salt medium to the rare earth molten salt electrolytic slag to react to obtain a reaction product, then washing with water, and performing solid-liquid separation to obtain a fluorine-containing alkaline leaching solution;

[0013] The fluorine-containing alkaline leaching solution is evaporated and concentrated;

[0014] The concentrated fluorine-containing alkaline leaching solution is mixed with the aluminum sulfate solution to obtain a mixed colloidal solution;

[0015] Sulfuric acid solution and sodium hydroxide solution are added dropwise to the mixed colloidal solution, and after reaction, cryolite is obtained by crystallization.

[0016] Preferably, the fluorine-containing alkaline immersion liquid is evaporated and concentrated so that the mass concentration of fluorine is 2 to 4 g / L;

[0017] The concentration of the aluminum sulfate solution is 0.5-1.0 mol / L.

[0018] Preferably, in the step of mixing the concentrated fluorine-containing alkaline leaching solution with the aluminum sulfate solution, the molar ratio of fluorine to aluminum sulfate in the fluorine-containing alkaline leaching solution is (6 to 6.5):(1 to 1.5);

[0019] The volume ratio of the concentrated fluorine-containing alkaline leaching solution and the aluminum sulfate solution is (200-210):(10-15).

[0020] Preferably, the mass concentration of the sulfuric acid solution is 30-60%;

[0021] The concentration of the sodium hydroxide solution is 0.1-0.3 g / L.

[0022] Preferably, sulfuric acid solution and sodium hydroxide solution are added dropwise to the mixed colloidal solution to adjust the pH of the mixed colloidal solution to 5.5-9, and the mixed colloidal solution is reacted at 30-90° C. for 10-30 minutes, and then crystallized to obtain cryolite.

[0023] Preferably, sulfuric acid solution and sodium hydroxide solution are added dropwise to the mixed colloidal solution to adjust the pH value of the mixed colloidal solution to 7, and the mixed colloidal solution is reacted at 30° C. for 10 to 30 minutes before crystallization to obtain cryolite.

[0024] Preferably, the molar ratio of fluorine to aluminum sulfate in the fluorine-containing alkaline immersion solution is 6:1.

[0025] Preferably, a sub-molten salt medium is added to the rare earth molten salt electrolytic slag, reacted at 120-130° C. to obtain a reaction product, which is then washed with water and solid-liquid separated to obtain a fluorine-containing alkaline leaching solution;

[0026] The sub-molten salt medium is an alkali-water mixed solution, and the mass concentration of the sub-molten salt medium is 50-80%; the mass ratio of the rare earth molten salt electrolytic slag to the alkali in the sub-molten salt medium is 1:(4-20).

[0027] Preferably, the base is at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate and sodium peroxide.

[0028] Preferably, the fluorine-containing alkaline immersion solution includes the following elements in mass concentrations: Na 15-15.5 g / L, Al 0.3-0.4 mg / L, F 2-2.5 g / L, Si 0.2-0.25 g / L.

[0029] The method of preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution of the present invention has the following beneficial effects compared with the prior art:

[0030] The method for preparing cryolite by utilizing alkaline leaching solution of rare earth molten salt electrolytic slag of the present invention prepares cryolite by using alkaline fluorine-containing waste liquid and aluminum sulfate, which is not only a supplement to the resource technology of fluorine-containing wastewater, but also an important link of the full recovery of valuable metals in rare earth molten salt electrolytic slag. The synthesis process is simple to operate, low in energy consumption, low in raw material cost, easy to realize industrialization, and has certain economic benefits; the mass percentage of cryolite prepared by the present invention meets the requirements of GB / T4291-2017, and the recovery rate of fluorine element in alkaline fluorine-containing wastewater and aluminum element added later reaches more than 95%, with a high recovery rate. The present invention has the advantages of being environmentally friendly, simple in process equipment, and high in recovery rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the prior art descriptions. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 The present invention is a schematic diagram of the preparation process of preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution. DETAILED DESCRIPTION

[0033] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0034] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of the embodiments. In addition, in the description of the present application, the term "including" means "including but not limited to". Various embodiments of the present invention may be in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be understood as a rigid limitation on the scope of the present invention; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the numbered ranges, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated in this article, it is meant to include any cited numbers (fractions or integers) within the indicated range.

[0035] The present application provides a method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution, comprising the following steps:

[0036] S1, adding a sub-molten salt medium to the rare earth molten salt electrolytic slag, reacting to obtain a reaction product, then washing with water, and separating the solid and liquid to obtain a fluorine-containing alkaline leaching solution;

[0037] S2, evaporating and concentrating the fluorine-containing alkaline leaching solution;

[0038] S3, mixing the concentrated fluorine-containing alkaline leaching solution with the aluminum sulfate solution to obtain a mixed colloidal solution;

[0039] S4. Add sulfuric acid solution and sodium hydroxide solution dropwise into the mixed colloidal solution, and after reaction, crystallize to obtain cryolite.

[0040] In some embodiments, the fluorine-containing alkaline immersion liquid is evaporated and concentrated to obtain a fluorine mass concentration of 2 to 4 g / L.

[0041] In some embodiments, aluminum sulfate 18hydrate is mixed with water and fully dissolved to obtain a 0.5-1.0 mol / L aluminum sulfate solution.

[0042] In some embodiments, in the step of mixing the concentrated fluorine-containing alkaline solution with the aluminum sulfate solution, the molar ratio of fluorine to aluminum sulfate in the fluorine-containing alkaline solution is (6-6.5):(1-1.5). Preferably, the molar ratio of fluorine to aluminum sulfate in the fluorine-containing alkaline solution is 6:1.

[0043] In some embodiments, the volume ratio of the concentrated fluorine-containing alkaline leaching solution and the aluminum sulfate solution is (200-210):(10-15), preferably, the volume ratio of the concentrated fluorine-containing alkaline leaching solution and the aluminum sulfate solution is 200:10.

[0044] In some embodiments, concentrated sulfuric acid is diluted to prepare a dilute sulfuric acid solution with a mass concentration of 30-60%.

[0045] In some embodiments, the concentration of the sodium hydroxide solution is 0.1-0.3 g / L.

[0046] In some embodiments, sulfuric acid solution and sodium hydroxide solution are added dropwise to a mixed colloidal solution to adjust the pH of the mixed colloidal solution to 5.5-9, and react at 30-90°C for 10-30 minutes to obtain cryolite, which is then filtered and dried to obtain the finished cryolite.

[0047] In some embodiments, sulfuric acid solution and sodium hydroxide solution are added dropwise to a mixed colloidal solution to adjust the pH value of the mixed colloidal solution to 7, and reacted at 30°C for 10 to 30 minutes to obtain cryolite, which is then filtered and dried to obtain the finished cryolite.

[0048] In some embodiments, a sub-molten salt medium is added to rare earth molten salt electrolytic slag, reacted at 120-130° C. to obtain a reaction product, washed with water, and solid-liquid separated to obtain a fluorine-containing alkaline leaching solution;

[0049] The sub-molten salt medium is an alkali-water mixed solution, and the mass concentration of the sub-molten salt medium is 50-80%; the mass ratio of rare earth molten salt electrolytic slag to the alkali in the sub-molten salt medium is 1:(4-20); the alkali is at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and sodium peroxide.

[0050] In some embodiments, the fluorine-containing alkaline immersion solution includes the following elements in mass concentrations: Na 15-15.5 g / L, Al 0.3-0.4 mg / L, F 2-2.5 g / L, Si 0.2-0.25 g / L, and the pH value of the fluorine-containing alkaline immersion solution is ≥13.

[0051] Preferably, the fluorine-containing alkaline immersion solution includes the following elements in mass concentrations: Na 15.3 g / L, Al 0.33 mg / L, F2.3 g / L, Si 0.22 g / L, and the pH value of the fluorine-containing alkaline immersion solution is ≥13.

[0052] For further reference, Figure 1 As shown, it is a schematic diagram of the preparation process of cryolite using rare earth molten salt electrolytic slag alkaline leaching solution of the present invention. Sub-molten salt medium is added to rare earth molten salt electrolytic slag, reacted to the reaction product, then washed with water, filtered to obtain rare earth hydroxide and alkaline fluorine-containing liquid (i.e., fluorine-containing alkaline leaching solution), evaporated and concentrated to obtain concentrated alkaline fluorine-containing liquid, added with aluminum sulfate solution to obtain colloidal turbid liquid (i.e., mixed colloidal solution), then added with sulfuric acid solution and sodium hydroxide solution, crystallized, and filtered to obtain cryolite and low-fluorine solution.

[0053] The present invention first decomposes rare earth molten salt slag by sub-molten salt technology, separates rare earth and fluorine therein by water washing, obtains rare earth hydroxide solid and alkaline solution of fluorine, and then concentrates the fluorine in the enriched solution by simple evaporation to obtain a high-alkali fluorine-containing solution. The fluorine-containing solution is introduced into the aluminum ions necessary for synthesizing cryolite by adding aluminum sulfate solution, and the pH value, component ratio, temperature and reaction time are adjusted to finally prepare high molecular ratio cryolite, thereby realizing the resource utilization of fluorine in molten salt slag. The present invention can efficiently recover rare earth in rare earth molten salt electrolytic slag, and can also efficiently recover fluorine in the slag, with a fluorine recovery rate of more than 95%. The process is simple, only simple impurity removal is required, and gradient separation of various ions is not required. It is not easy to generate acidic waste gas under alkaline conditions, and it is safer. The prepared cryolite meets the requirements of national standards, has a simple process, is easy to operate, has low cost and has significant environmental and economic value.

[0054] The following further illustrates the method for preparing cryolite using rare earth molten salt electrolytic slag alkali leaching solution of the present application with specific examples. This section further illustrates the content of the present invention in conjunction with specific examples, but should not be construed as limiting the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0055] The mass fractions of the elements in the rare earth molten salt electrolytic slag in the following examples are shown in Table 1:

[0056] Table 1-Mass fraction of each element in rare earth molten salt electrolytic slag

[0057]

[0058] Example 1

[0059] The present application provides a method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution, comprising the following steps:

[0060] S1. Add a sub-molten salt medium to the rare earth molten salt electrolytic slag, react at 125°C to obtain a reaction product, then wash with water, and separate the solid and liquid to obtain a fluorine-containing alkaline leaching solution; the sub-molten salt medium is an alkaline water mixed solution, the mass concentration of the sub-molten salt medium is 60%, and the alkali is sodium hydroxide; the mass ratio of the rare earth molten salt electrolytic slag to the alkali in the sub-molten salt medium is 1:10; the fluorine-containing alkaline leaching solution includes the following elements in mass concentrations: Na15.3g / L, Al0.33mg / L, F2.3g / L, Si0.22g / L, and the pH value of the fluorine-containing alkaline leaching solution is ≥13;

[0061] S2, evaporating and concentrating the fluorine-containing alkaline leaching solution so that the mass concentration of fluorine is 2.33 g / L;

[0062] S3, mixing 200 mL of fluorine-containing alkaline leaching solution with a fluorine mass concentration of 2.33 g / L with 10 mL of 0.4 mol / L aluminum sulfate solution, and shaking for 1 min to obtain a mixed colloidal solution;

[0063] S4. Add 50% sulfuric acid solution and 0.1g / L sodium hydroxide solution into the mixed colloidal solution to adjust the pH value to 7, place in an oil bath at 30°C and react for 15 minutes to crystallize into cryolite, filter, and dry at 120°C for 1 hour to obtain the finished cryolite.

[0064] Example 2

[0065] The present application provides a method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution, comprising the following steps:

[0066] S1. Add a sub-molten salt medium to the rare earth molten salt electrolytic slag, react at 125°C to obtain a reaction product, then wash with water, and separate the solid and liquid to obtain a fluorine-containing alkaline leaching solution; the sub-molten salt medium is an alkaline water mixed solution, the mass concentration of the sub-molten salt medium is 60%, and the alkali is sodium hydroxide; the mass ratio of the rare earth molten salt electrolytic slag to the alkali in the sub-molten salt medium is 1:10; the fluorine-containing alkaline leaching solution includes the following elements in mass concentrations: Na 15.3g / L, Al 0.33mg / L, F 2.3g / L, Si 0.22g / L, and the pH value of the fluorine-containing alkaline leaching solution is ≥13;

[0067] S2, evaporating and concentrating the fluorine-containing alkaline leaching solution so that the mass concentration of fluorine is 2.33 g / L;

[0068] S3, mixing 200 mL of fluorine-containing alkaline leaching solution with a fluorine mass concentration of 2.33 g / L with 10 mL of 0.4 mol / L aluminum sulfate solution, and shaking for 3 minutes to obtain a mixed colloidal solution;

[0069] S4. Add 50% sulfuric acid solution and 0.1g / L sodium hydroxide solution into the mixed colloidal solution to adjust the pH value to 7, place in an oil bath at 30°C and react for 20 minutes to crystallize into cryolite, filter, and dry at 120°C for 1 hour to obtain the finished cryolite.

[0070] Example 3

[0071] The present application provides a method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution, comprising the following steps:

[0072] S1. Add a sub-molten salt medium to the rare earth molten salt electrolytic slag, react at 125°C to obtain a reaction product, then wash with water, and separate the solid and liquid to obtain a fluorine-containing alkaline leaching solution; the sub-molten salt medium is an alkaline water mixed solution, the mass concentration of the sub-molten salt medium is 60%, and the alkali is sodium hydroxide; the mass ratio of the rare earth molten salt electrolytic slag to the alkali in the sub-molten salt medium is 1:10; the fluorine-containing alkaline leaching solution includes the following elements in mass concentrations: Na15.3g / L, Al0.33mg / L, F2.3g / L, Si0.22g / L, and the pH value of the fluorine-containing alkaline leaching solution is ≥13;

[0073] S2, evaporating and concentrating the fluorine-containing alkaline leaching solution so that the mass concentration of fluorine is 2.33 g / L;

[0074] S3, mixing 200 mL of fluorine-containing alkaline leaching solution with a fluorine mass concentration of 2.33 g / L with 10 mL of 0.4 mol / L aluminum sulfate solution, and shaking for 3 minutes to obtain a mixed colloidal solution;

[0075] S4. Add 50% sulfuric acid solution and 0.1g / L sodium hydroxide solution into the mixed colloidal solution to adjust the pH value to 7, place in an oil bath at 30°C and react for 25 minutes to crystallize into cryolite, filter, and dry at 120°C for 1 hour to obtain the finished cryolite.

[0076] Cryolite was prepared according to the methods in Examples 1 to 3, and the fluorine recovery rate, aluminum recovery rate and mass percentage of each element in the cryolite were calculated. The results are shown in Table 2. Specifically, the fluorine and aluminum ion concentrations of the solution after preparing the cryolite were detected, and the fluorine recovery rate and aluminum recovery rate were calculated by comparing the concentrations of the ions with those in the fluorine-containing alkaline leaching solution.

[0077] Table 2 - Fluorine recovery rate, aluminum recovery rate and mass percentage of each element in cryolite in different embodiments

[0078]

[0079] It can be seen from Table 2 that the mass percentages of cryolite obtained in Examples 1 to 3 of the present invention all meet the requirements of GB / T4291-2017, and the recovery rates of fluorine element in alkaline fluorine-containing wastewater and aluminum element added later reach more than 95%, with a high recovery rate. The present invention has the advantages of being environmentally friendly, simple process equipment, and high recovery rate. It is not only a supplement to the fluorine-containing wastewater resource technology, but also an important part of the full recovery of valuable metals in rare earth molten salt electrolytic slag.

[0080] The above description is only a preferred embodiment of the present invention and is 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 protection scope of the present invention.

Claims

1. A method for preparing cryolite using rare earth molten salt electrolytic slag alkali leaching solution, characterized in that: The following steps are involved: Adding a sub-molten salt medium to the rare earth molten salt electrolytic slag to react to obtain a reaction product, then washing with water, and performing solid-liquid separation to obtain a fluorine-containing alkaline leaching solution; The fluorine-containing alkaline leaching solution is evaporated and concentrated; The concentrated fluorine-containing alkaline leaching solution is mixed with the aluminum sulfate solution to obtain a mixed colloidal solution; Sulfuric acid solution and sodium hydroxide solution are added dropwise to the mixed colloidal solution, and after reaction, cryolite is obtained by crystallization.

2. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 1, characterized in that: The fluorine-containing alkaline leaching solution is evaporated and concentrated to make the mass concentration of fluorine be 2 to 4 g / L; The concentration of the aluminum sulfate solution is 0.5-1.0 mol / L.

3. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 2, characterized in that: In the step of mixing the concentrated fluorine-containing alkaline leaching solution with the aluminum sulfate solution, the molar ratio of fluorine to aluminum sulfate in the fluorine-containing alkaline leaching solution is (6 to 6.5):(1 to 1.5); The volume ratio of the concentrated fluorine-containing alkaline leaching solution and the aluminum sulfate solution is (200-210):(10-15).

4. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 1, characterized in that: The mass concentration of the sulfuric acid solution is 30-60%; The concentration of the sodium hydroxide solution is 0.1-0.3 g / L.

5. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 1, characterized in that: The sulfuric acid solution and the sodium hydroxide solution are added dropwise to the mixed colloidal solution to adjust the pH value of the mixed colloidal solution to 5.5-9, and the mixed colloidal solution is reacted at 30-90° C. for 10-30 minutes, and then crystallized to obtain cryolite.

6. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 5, characterized in that: Sulfuric acid solution and sodium hydroxide solution are added dropwise to the mixed colloidal solution to adjust the pH value of the mixed colloidal solution to 7, and the mixed colloidal solution is reacted at 30° C. for 10 to 30 minutes, and then crystallized to obtain cryolite.

7. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution as claimed in claim 3, characterized in that: The molar ratio of fluorine to aluminum sulfate in the fluorine-containing alkaline leaching solution is 6:

1.

8. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 1, characterized in that: Adding a sub-molten salt medium to rare earth molten salt electrolytic slag, reacting at 120-130° C. to obtain a reaction product, then washing with water, and performing solid-liquid separation to obtain a fluorine-containing alkaline leaching solution; The sub-molten salt medium is an alkali-water mixed solution, and the mass concentration of the sub-molten salt medium is 50-80%; the mass ratio of the rare earth molten salt electrolytic slag to the alkali in the sub-molten salt medium is 1:(4-20).

9. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 8, characterized in that: The alkali is at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate and sodium peroxide.

10. The method for preparing cryolite by using rare earth molten salt electrolytic slag alkali leaching solution according to claim 1, characterized in that: The fluorine-containing alkaline immersion solution includes the following elements in mass concentrations: Na 15-15.5 g / L, Al 0.3-0.4 mg / L, F 2-2.5 g / L, and Si 0.2-0.25 g / L.

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