Method for recovering cryolite

Abstract

The invention discloses a method for recycling cryolite, which comprises the following steps of: 1, mixing and molding electrolytic aluminum waste residue and a solid decomposing agent, and reacting under a heating condition to obtain fluorine-containing mixed gas and a defluorination material; step 2, carrying out circulating absorption on the fluorine-containing mixed gas to obtain an absorption mixture, and staying and desiliconizing to obtain a fluorine absorption liquid and white carbon black; 3, the defluorinated material obtained in the step 1 is leached with water, and leaching liquid containing sodium salt and aluminum salt and carbon residues are obtained after liquid-solid separation; and 4, mixing the leachate obtained in the step 3 with the fluorine absorption liquid obtained in the step 2, and then carrying out reactive crystallization to obtain cryolite crystals and a decomposing agent solution. The cryolite prepared by the method is adjustable in property, the decomposing agent can be recycled, the process is simple, and the conditions are mild.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of industrial hazardous waste resource recycling. Specifically, it is a method for recycling cryolite. BACKGROUND

[0002] Electrolytic aluminum waste mainly includes anode dross and waste cathode. The anode dross is generated due to the reasons such as uneven oxidation, electrolyte impact, etc. during the production process of carbon anode, and generally 3-5 kg of anode dross is generated per ton of electrolytic aluminum, of which the electrolyte accounts for 70-80%, and the rest is carbon. The electrolytic aluminum waste cathode is generated after the electrolytic tank is replaced due to electrolyte erosion and aging after running for 5-7 years, and the main component is carbon material, accounting for 50-70%, and the rest is fluorine-containing electrolyte. However, due to the long running time, it may contain other inorganic compounds and trace amounts of cyanide (about 0.2%), and generally 30-50 kg of waste cathode is generated per ton of electrolytic aluminum. In 2024, the electrolytic aluminum production was 44.05 million tons, an increase of 4.6% year-on-year, and the amount of electrolytic aluminum waste generated was about 1.76 million tons. Without measures, landfill or storage will cause the release of fluorides and cyanides, and the toxicity of a large amount of soluble fluorine components and trace amounts of cyanide components will cause irreparable damage to the soil or water body.

[0003] During the production of electrolytic aluminum, part of the fluorides will escape at a high temperature of 950-970 °C. According to statistics, 20-25 kg of aluminum fluoride needs to be supplemented per ton of electrolytic aluminum. If the fluorides in the electrolytic aluminum waste are supplemented into the electrolytic tank in the form of cryolite, the consumption of aluminum fluoride can be reduced. Therefore, the preparation of cryolite from electrolytic aluminum waste not only realizes the internal circulation of fluorine-containing materials, but also solves the problem of fluorine pollution, and has significant economic and social benefits.

[0004] In the prior art, patent number CN107364880B mixes electrolytic aluminum waste tank lining with bisulfate, and after calcination at 900-1000 °C, reacts with aluminum oxide to prepare aluminum fluoride. Since the melting points of bisulfates are relatively low, problems such as sticking or scarring of the pyrolysis furnace make the heat transfer efficiency of the reaction poor and even difficult to operate, and bisulfates will react with carbon to generate carbon dioxide at high temperatures. In the prior art, there is also a method for preparing cryolite by reacting fluorine-containing waste with concentrated sulfuric acid, but using liquid concentrated sulfuric acid as a reactant is dangerous and requires high equipment material quality. The carbon in the waste will generate carbon dioxide, causing resource waste, and the generated sodium sulfate and aluminum sulfate wastewater is large and cannot be treated, thereby causing new waste discharge problems. SUMMARY

[0005] To this end, the technical problem to be solved by the present application is to provide a method for recycling cryolite, which effectively recovers a cryolite product with high purity, controllable particle size and molecular ratio (molar ratio of sodium fluoride and aluminum fluoride in cryolite), and has mild process operating conditions, recyclable decomposition agent, and no new waste liquid and solid waste.

[0006] To solve the above technical problems, the present application provides the following technical solutions:

[0007] A method for recycling cryolite, comprising the following steps:

[0008] Step one, mixing electrolytic aluminum waste residue and solid decomposition agent into a shape, and then reacting under heating conditions to obtain fluorine-containing mixed gas and defluorination material;

[0009] Step two, cyclically absorbing the fluorine-containing mixed gas to obtain an absorption mixture, and then obtaining a fluorine absorption liquid and white carbon black after desiliconization;

[0010] Step three, leaching the defluorination material obtained in step one with water, and then obtaining a leaching liquid containing sodium salt and aluminum salt and carbon residue after liquid-solid separation;

[0011] Step four, mixing the leaching liquid obtained in step three with the fluorine absorption liquid obtained in step two, and then reacting and crystallizing, and then obtaining cryolite crystals and decomposition agent solution after liquid-solid separation.

[0012] The present application reacts under mild conditions (not higher than 400 DEG C), which is much lower than the sintering method combined disposal process (about 950 DEG C) or the American high-temperature hydrolysis method (about 1200 DEG C), so that the target product cryolite can be obtained, the efficient conversion and separation of fluorides in electrolytic aluminum waste residue are realized, the process is simple, and all four main elements of carbon, fluorine, sodium and aluminum in electrolytic aluminum waste residue can be recycled at the same time. Except for mechanical loss, theoretically, there is no consumption of solid decomposition agent, the recycling and utilization of solid hazardous waste can be realized through energy consumption, and the production cost is low.

[0013] Specifically, electrolytic aluminum waste residue (fluorides mainly including carbon and cryolite) is mixed with a solid decomposition agent, and then reacts under heating conditions to obtain defluorination material including sodium salt and aluminum salt and fluorine-containing mixed gas. Since the electrolytic aluminum waste residue of the present application contains silicon dioxide, it will react with the solid decomposition agent and fluorides, so that the fluorine-containing mixed gas includes silicon tetrafluoride gas, the excess part of the solid decomposition agent of the present application generates ammonia gas in the thermal decomposition process, is dissolved in the absorption liquid of the fluorine-containing mixed gas to maintain an alkaline chemical environment, and then facilitates the hydrolysis of silicon tetrafluoride gas in the circulating absorber and the generation of ammonium fluorosilicate; and at the same time, the ammonium fluorosilicate reacts with ammonia water in the desiliconization process to convert into white carbon black and ammonium fluoride which is easy to filter. Without the need for additional addition of chemical reagents, the quality of the final product cryolite is ensured.

[0014] The method for recycling cryolite, in step one: the heating temperature is 200-400℃. At the temperature of 200-400℃, the reaction kiln and the like can be prevented from being difficult to operate due to adhesion and scabbing, the requirement for the equipment material is lower, and the energy consumption is lower.

[0015] The method for recycling cryolite, in step one: the solid decomposing agent is one of ammonium sulfate and ammonium chloride, or a mixture of the two; the reference amount of the solid decomposing agent is that the molar ratio of ammonium ions to fluorine ions in the reaction raw material is 1:1, and the amount of the solid decomposing agent in the pyrolysis process is 5-50% more than the reference amount; the excess part of the solid decomposing agent makes the ammonia gas generated in the thermal decomposition dissolve in the fluorine-containing mixed gas absorption liquid, so as to maintain the alkaline chemical absorption environment.

[0016] The method for recycling cryolite, in step one: the electrolytic aluminum waste residue and the solid decomposing agent are mixed, and an oxidizing agent is further added, the oxidizing agent being one or both of ammonium persulfate and sodium persulfate; the amount of the oxidizing agent is 1-2 times the amount of cyanide in the electrolytic aluminum waste residue. The addition of the oxidizing agent reacts with the cyanide in the electrolytic aluminum waste residue to eliminate the toxicity.

[0017] The method for recycling cryolite, in step one: the electrolytic aluminum waste residue and the solid decomposing agent are mixed and uniformly ground to obtain a mixed powder, and the mixed powder is made into a 5-10mm shaped material, the shaped material being a strip-shaped, block-shaped or granular reaction material, which is then sent to a thermal decomposition process and reacted at 200-400℃. Specifically, the mixed powder is extruded into a 5-10mm strip / block-shaped material by a molding machine, or the mixed powder is granulated to obtain 5-10mm particles; the generated ammonium fluoride gas is easily escaped, and the carbon and electrolyte in the electrolytic aluminum waste residue are prevented from escaping with the gas flow, so that the gas phase entrainment problem is solved.

[0018] The method for recycling cryolite, in step two: the fluorine-containing mixed gas is absorbed by the circulating absorption liquid, and the temperature in the absorption process is controlled to be 25-55℃.

[0019] The method for recycling cryolite, in step two: the gas absorption medium is water.

[0020] The method for recycling cryolite, in step two: the fluorine absorption liquid is an ammonium fluoride solution, and the absorption is circulated until the mass fraction of ammonium fluoride in the ammonium fluoride solution is 20%-45 wt%.

[0021] The method for recycling cryolite in the above, in step four: the obtained decomposing agent solution is evaporated and crystallized, and after liquid-solid separation, the decomposing agent crystals and the decomposing agent mother liquor are obtained, the decomposing agent crystals are recycled to step one, and the decomposing agent mother liquor is returned to step four for preparing process to adjust the cryolite molecular ratio. The recycling of the decomposing agent solution realizes zero emission of the reaction medium.

[0022] The method for recycling cryolite in the above, in step four: when the cryolite is synthesized, the molar ratio of fluorine, aluminum and sodium in the synthetic solution after mixing the leaching solution, the fluorine absorption solution and the decomposing agent mother liquor is 6:1:(3-6).

[0023] The method for recycling cryolite in the above, in step three: in the leaching solution, the concentration of sodium salt is 150-190 g / L, and the concentration of aluminum salt is 150-180 g / L.

[0024] The method for recycling cryolite in the above, in step three: after the carbon residue is leached with the hydrochloric acid solution, high-purity carbon can be obtained, which can be used for preparing electrolytic aluminum anode or cathode carbon block.

[0025] The technical scheme of the present application achieves the following beneficial technical effects:

[0026] 1. The present application reacts under mild conditions of not higher than 400 DEG C, which is much lower than the sintering method combined disposal process (about 950 DEG C) or the American high-temperature hydrolysis method (about 1200 DEG C), so that the target product cryolite can be obtained, realizing efficient conversion and separation of fluorides in electrolytic aluminum waste residue, and the process is simple; and all four main elements of carbon, fluorine, sodium and aluminum in the electrolytic aluminum waste residue can be recycled at the same time, realizing efficient conversion of elements.

[0027] 2. After the electrolytic aluminum waste residue (fluorides mainly including carbon and cryolite) is mixed with the solid decomposing agent and reacts under heating conditions, defluorination materials including sodium salt and aluminum salt and fluorine-containing mixed gas are obtained. Since the electrolytic aluminum waste residue of the present application contains silicon dioxide, it will react with the solid decomposing agent and fluorides, so that the fluorine-containing mixed gas includes silicon tetrafluoride gas, and the excess part of the solid decomposing agent generates ammonia gas in the thermal decomposition process, is dissolved in the fluorine-containing mixed gas absorption solution to maintain an alkaline chemical environment, which is conducive to the hydrolysis of silicon tetrafluoride gas in the circulating absorber and the generation of ammonium fluorosilicate; and at the same time, the ammonium fluorosilicate reacts with ammonia water in the remaining desiliconization process to convert into white carbon black and ammonium fluoride which are easy to filter. Without additional addition of chemical reagents, the quality of the final product cryolite is ensured.

[0028] 3. The decomposing agent mother liquor obtained in the present application is returned to the cryolite preparation process to adjust the cryolite molecular ratio, so that the molar ratio of sodium fluoride and aluminum fluoride in the cryolite is controllable; and after the by-product carbon residue is removed by hydrochloric acid, high-purity carbon can be obtained, which can be returned to the electrolytic aluminum process for continuous use.

[0029] 4. In addition to mechanical loss, the application theoretically has no consumption of solid decomposition agent, and the resource recycling of solid hazardous waste can be realized through energy consumption, and the production cost is low. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The process flow chart of the application. DETAILED DESCRIPTION

[0031] The main reaction equation of the application is as follows:

[0032]

[0033] Example 1

[0034] The method for recycling cryolite of the present application comprises the following steps:

[0035] (1) The electrolytic aluminum waste residue is mixed with ammonium sulfate (the stoichiometric ratio of the amount of substance of ammonium ion to the amount of substance of fluorine ion in the reaction raw material is 1:1, and the amount of addition of ammonium sulfate in the present example is 40% more than the stoichiometric amount) and ammonium persulfate as an oxidizing agent (the amount of substance of ammonium persulfate is 1 times the amount of substance of cyanide in the electrolytic aluminum waste residue), and the mixture is uniformly mixed to obtain a mixed powder. The mixed powder is then extruded into a 5-10mm strip / block by a molding machine, and is reacted at a heating condition of 200℃ for 4h to obtain a fluorine-containing mixed gas and a defluorinated material; the excess ammonium sulfate is decomposed to generate ammonia gas which is dissolved in the mixed gas absorption liquid to maintain an alkaline absorption environment.

[0036] (2) The fluorine-containing mixed gas is subjected to cyclic absorption by water, and the temperature during the absorption process is controlled at 25℃ to obtain an absorption mixture, and at the same time, the fluorine absorption liquid with a mass concentration of 30% of ammonium fluoride and white carbon black are obtained after desiliconization.

[0037] (3) The defluorinated material obtained in step (1) is leached with 90℃ hot water for 2h to obtain a leaching solution containing aluminum sulfate, sodium sulfate and carbon residue, and the concentration of sodium sulfate in the leaching solution is 150g / L, and the concentration of aluminum sulfate is 170g / L.

[0038] (4) The synthesis material liquid obtained by mixing the leaching solution, the fluorine absorption liquid and the ammonium sulfate mother liquor obtained in step (5) is reacted at 90℃ under stirring for 2h to obtain cryolite and an ammonium sulfate solution, and the ratio of the amount of substance of fluorine, aluminum and sodium in the synthesis material liquid is 6:1:3.5.

[0039] (5) The ammonium sulfate solution is evaporated and concentrated to crystallize, and after liquid-solid separation, ammonium sulfate crystals and ammonium sulfate mother liquor are obtained, the ammonium sulfate crystals are returned to step (1) for recycling, and the ammonium sulfate mother liquor is returned to step (5) to adjust the ratio of fluorine, aluminum and sodium.

[0040] (7) The carbon residue is dissolved in hydrochloric acid to dissolve the calcium sulfate and other insoluble impurities therein, and high-purity carbon is obtained for producing electrolytic aluminum anode and cathode carbon block.

[0041] Example 2

[0042] The method for recycling cryolite in this embodiment comprises the following steps:

[0043] (1) The electrolytic aluminum waste residue is mixed with ammonium sulfate (the reference amount of the solid decomposition agent is 1:1 of the amount of substance of ammonium ion to fluorine ion in the reaction raw material, and the amount of ammonium sulfate added in this embodiment is 50% more than the reference amount) and ammonium persulfate as an oxidizing agent (the amount of substance of ammonium persulfate is 1 times the amount of substance of cyanide in the electrolytic aluminum waste residue), to obtain a mixed powder, and then the mixed powder is extruded into a 5-10mm strip / block by a molding machine, and reacted for 4h under heating at 220℃ to obtain a fluorine-containing mixed gas and a defluorination material; the excess ammonium sulfate is decomposed to generate ammonia gas which is dissolved in the mixed gas absorption liquid to maintain an alkaline absorption environment.

[0044] (2) The fluorine-containing mixed gas is cyclically absorbed by water to obtain an absorption mixture, and the absorption process is controlled at a temperature of 30℃, and at the same time, the defluorination residue is obtained to obtain a white carbon black and a fluorine absorption liquid with a mass concentration of 30% of ammonium fluoride.

[0045] (3) The defluorination material obtained in step (1) is immersed in 100℃ hot water for 1h to obtain an immersion liquid containing aluminum sulfate, sodium sulfate and carbon residue, and the concentration of sodium sulfate in the immersion liquid is 160g / L, and the concentration of aluminum sulfate is 170g / L.

[0046] (4) The synthesis material liquid obtained by mixing the immersion liquid, the fluorine absorption liquid and the ammonium sulfate mother liquor obtained in step (5) is reacted for 2h under stirring at 90℃, and cryolite and an ammonium sulfate solution are obtained by the reaction, and the amount of substance ratio of fluorine, aluminum and sodium in the synthesis material liquid is 6:1:4.

[0047] (5) The ammonium sulfate solution is evaporated and concentrated to crystallize, and ammonium sulfate crystals and an ammonium sulfate mother liquor are obtained after liquid-solid separation, the ammonium sulfate crystals are returned to step (1) for recycling, and the ammonium sulfate mother liquor is returned to step (4) to adjust the fluorine, aluminum and sodium ratio.

[0048] (7) The carbon residue is dissolved in hydrochloric acid to dissolve the calcium sulfate and other insoluble impurities therein, and high-purity carbon is obtained for producing electrolytic aluminum anode and cathode carbon block.

[0049] Example 3

[0050] The method for recycling cryolite in this embodiment comprises the following steps:

[0051] (1) The electrolytic aluminum waste residue is mixed evenly with ammonium sulfate (the standard addition amount of solid decomposition agent is the ratio of the amount of ammonium ions to fluoride ions in the reaction raw materials is 1:1, and in this embodiment, the addition amount of ammonium sulfate is 50% excess on the basis of the standard addition amount) and ammonium persulfate as oxidant (the amount of ammonium persulfate is 1 times the amount of cyanide in the electrolytic aluminum waste residue) to obtain a mixed powder. The mixed powder is then extruded into strips / blocks of 5~10mm by a molding machine and reacted under heating conditions of 240℃ for 4h to obtain a fluorine-containing mixed gas and defluorinated material. The ammonia gas generated by the decomposition of excess ammonium sulfate dissolves in the mixed gas absorption liquid to maintain an alkaline absorption environment.

[0052] (2) The fluorine-containing mixed gas is absorbed by water to obtain an absorption mixture. The temperature of the absorption process is controlled at 25°C. At the same time, after desiliconization, a fluorine absorption liquid with an ammonium fluoride mass concentration of 20% and white carbon black are obtained.

[0053] (3) After leaching the defluorinated material obtained in step (1) with hot water at 100℃ for 2 hours, a leachate containing aluminum sulfate and sodium sulfate and carbon residue are obtained. The concentration of sodium sulfate in the leachate is 150 g / L and the concentration of aluminum sulfate is 180 g / L.

[0054] (4) The synthetic solution obtained by mixing the leachate, fluorine absorption solution and ammonium sulfate mother liquor obtained in step (5) is reacted at 90°C for 2 hours with stirring to obtain cryolite and ammonium sulfate solution. The molar ratio of fluorine, aluminum and sodium in the synthetic solution is 6:1:4.

[0055] (5) The ammonium sulfate solution is evaporated, concentrated and crystallized. After liquid-solid separation, ammonium sulfate crystals and ammonium sulfate mother liquor are obtained. The ammonium sulfate crystals are returned to step (1) for recycling, and the ammonium sulfate mother liquor is returned to step (4) to adjust the ratio of fluorine, aluminum and sodium.

[0056] (6) After dissolving the insoluble impurities such as calcium sulfate in the carbon slag with hydrochloric acid, high-purity carbon is obtained for the production of electrolytic aluminum anode and cathode carbon blocks.

[0057] Example 4

[0058] This embodiment describes a method for recycling cryolite, comprising the following steps:

[0059] (1) The electrolytic aluminum waste residue is mixed with ammonium chloride (the standard addition amount of solid decomposition agent is the ratio of the amount of ammonium ions to fluoride ions in the reaction raw materials is 1:1, and in this embodiment, the addition amount of ammonium chloride is 5% excess on the basis of the standard addition amount) and ammonium persulfate as oxidant (the amount of ammonium persulfate is 1 times the amount of cyanide in the electrolytic aluminum waste residue) to obtain a mixed powder. The mixed powder is then extruded into strips / blocks of 5~10mm by a molding machine and reacted under heating conditions of 260℃ for 4h to obtain a fluorine-containing mixed gas and defluorinated material. The ammonia gas generated by the decomposition of excess ammonium chloride dissolves in the mixed gas absorption liquid to maintain an alkaline absorption environment.

[0060] (2) The fluorine-containing mixed gas is absorbed by water to obtain an absorption mixture. The temperature of the absorption process is controlled at 30°C. At the same time, after desiliconization, a fluorine absorption liquid with a mass concentration of 20% ammonium fluoride and white carbon black are obtained.

[0061] (3) After leaching the defluorinated material obtained in step (1) with hot water at 90℃ for 2 hours, a mixed solution containing aluminum chloride and sodium chloride and carbon residue are obtained. The concentration of sodium chloride in the leachate is 160 g / L and the concentration of aluminum chloride is 170 g / L.

[0062] (4) The synthetic solution obtained by mixing the leachate, fluorine absorbent and ammonium chloride mother liquor obtained in step (5) is reacted at 90°C for 2 hours with stirring to obtain cryolite and ammonium chloride solution. The molar ratio of fluorine, aluminum and sodium in the synthetic solution is 6:1:5.

[0063] (5) The ammonium chloride solution is evaporated, concentrated and crystallized. After solid-liquid separation, ammonium chloride crystals and ammonium chloride mother liquor are obtained. The ammonium chloride crystals are returned to step (1) for recycling, and the ammonium chloride mother liquor is returned to step (4) to adjust the ratio of fluorine, aluminum and sodium.

[0064] (6) After dissolving the insoluble impurities such as calcium sulfate in the carbon slag with hydrochloric acid, high-purity carbon is obtained for the production of electrolytic aluminum anode and cathode carbon blocks.

[0065] Example 5

[0066] This embodiment of a method for recycling cryolite includes the following steps:

[0067] (1) The electrolytic aluminum waste residue is mixed with ammonium sulfate (the standard addition amount of solid decomposition agent is the ratio of the amount of ammonium ions to fluoride ions in the reaction raw materials is 1:1, and in this embodiment, the addition amount of ammonium sulfate is 50% excess on the basis of the standard addition amount) and sodium persulfate as oxidant (the amount of ammonium persulfate is 1 times the amount of cyanide in the electrolytic aluminum waste residue) to obtain a mixed powder. The mixed powder is then extruded into strips / blocks of 5~10mm by a molding machine and reacted at 400℃ for 4h to obtain a fluorine-containing mixed gas and defluorinated material. The ammonia gas generated by the decomposition of excess ammonium sulfate dissolves in the mixed gas absorption liquid to maintain an alkaline absorption environment.

[0068] (2) The fluorine-containing mixed gas is absorbed by water to obtain an absorption mixture. The temperature of the absorption process is controlled at 50°C. At the same time, after desiliconization, a fluorine absorption liquid with an ammonium fluoride mass concentration of 20% and white carbon black are obtained.

[0069] (3) After leaching the defluorinated material obtained in step (1) with hot water at 90℃ for 2 hours, a mixed solution containing aluminum sulfate and sodium sulfate and carbon residue are obtained. The concentration of sodium sulfate in the leachate is 190 g / L and the concentration of aluminum sulfate is 170 g / L.

[0070] (4) The synthetic solution obtained by mixing the leachate, fluorine absorbent and ammonium sulfate mother liquor obtained in step (5) is stirred at 90°C for 2 hours to obtain cryolite and ammonium sulfate solution. The molar ratio of fluorine, aluminum and sodium in the synthetic solution is 6:1:4.

[0071] (5) The ammonium sulfate solution is evaporated, concentrated and crystallized. After liquid-solid separation, ammonium sulfate crystals and ammonium sulfate mother liquor are obtained. The ammonium sulfate crystals are returned to step (1) for recycling, and the ammonium sulfate mother liquor is returned to step (4) to adjust the ratio of fluorine, aluminum and sodium.

[0072] (6) After dissolving the insoluble impurities such as calcium sulfate in the carbon slag with hydrochloric acid, high-purity carbon is obtained for the production of electrolytic aluminum anode and cathode carbon blocks.

[0073] Table 1. Quality analysis results of cryolite produced in Examples 1-2

[0074]

[0075] As shown in Table 1, this invention not only successfully recovered fluorides from electrolytic aluminum slag but also produced high-quality cryolite with a purity exceeding industry standards. The low SiO2 residue demonstrates that the ammonia decomposed using the solid decomposition agent achieved desilication. Simultaneously, the low sulfate residue and loss on ignition prove that this process effectively avoids product contamination problems associated with concentrated sulfuric acid processes. The high-purity cryolite obtained in this application can not only be recycled back into the electrolytic cell but also used as a commodity in other industries, creating greater economic value.

[0076] Table 2. Quality analysis results of graphite produced in Examples 1 and 2.

[0077]

[0078] As can be seen from Table 2, after dissolving the calcium sulfate impurities in the carbon slag with hydrochloric acid and drying it, the high-purity carbon obtained has fewer impurities and will not produce harmful gases such as sulfur dioxide in subsequent use. This proves that the process can achieve comprehensive resource recovery of materials, and has both economic and environmental benefits.

[0079] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method of recovering cryolite, characterized in that, The method comprises the following steps: Step 1: mixing electrolytic aluminum waste residue and solid decomposition agent, forming, and then reacting under heating to obtain fluorine-containing mixed gas and defluorination material; Step 2: cyclically absorbing the fluorine-containing mixed gas to obtain an absorption mixture, and then stopping desiliconization to obtain a fluorine absorption solution and white carbon black; Step 3: leaching the defluorination material obtained in Step 1 with water, and then performing liquid-solid separation to obtain a leaching solution containing sodium salt and aluminum salt and carbon residue; Step 4: mixing the leaching solution obtained in Step 3 with the fluorine absorption solution obtained in Step 2, and then performing reaction crystallization, and then performing liquid-solid separation to obtain cryolite crystals and decomposition agent solution.

2. A method of recovering cryolite according to claim 1, characterized in that, In Step 1: the heating temperature is 200-400℃.

3. A method of recovering cryolite according to claim 1, characterized in that, In Step 1: the solid decomposition agent is one of ammonium sulfate and ammonium chloride, or a mixture of the two; the reference amount of the solid decomposition agent is such that the molar ratio of ammonium ions to fluorine ions in the reaction raw materials is 1:1, and the amount of the solid decomposition agent added in the pyrolysis process is 5-50% more than the reference amount; the excess part of the solid decomposition agent makes the ammonia gas generated in the thermal decomposition dissolve in the fluorine-containing mixed gas absorption solution, thereby maintaining an alkaline chemical absorption environment.

4. A method of recovering cryolite according to claim 1, characterized in that, In Step 1: an oxidizing agent is also added when the electrolytic aluminum waste residue and the solid decomposition agent are mixed, and the oxidizing agent is one or both of ammonium persulfate and sodium persulfate; the amount of the oxidizing agent added is 1-2 times the amount of cyanide in the electrolytic aluminum waste residue.

5. A method of recovering cryolite according to claim 1, characterized in that, In Step 1: the electrolytic aluminum waste residue and the solid decomposition agent are mixed and ground uniformly to obtain a mixed powder, the mixed powder is then made into a formed material with a size of 5-10 mm, the formed material is a strip-shaped, block-shaped or granular reaction material, and then the formed material is sent to a thermal decomposition process for reaction at 200-400℃.

6. A method of recovering cryolite according to claim 1, characterized in that, In Step 2: the fluorine-containing mixed gas is absorbed by a circulating absorption solution, and the absorption process is controlled at a temperature of 25-55℃.

7. A method of recovering cryolite according to claim 1, characterized in that, In Step 2: the fluorine absorption solution is an ammonium fluoride solution, and the absorption is continued until the mass fraction of ammonium fluoride in the ammonium fluoride solution is 20%-45 wt%.

8. A method of recovering cryolite according to claim 1, characterized in that, In Step 4: the decomposition agent solution obtained is evaporated and crystallized, and then liquid-solid separation is performed to obtain decomposition agent crystals and decomposition agent mother liquor; the decomposition agent crystals are recycled in Step 1, and the decomposition agent mother liquor is returned to Step 4 for preparation to adjust the molecular ratio of cryolite.

9. A method of recovering cryolite according to claim 8, characterised in that, In Step 4: when the cryolite is synthesized, the molar ratio of fluorine, aluminum and sodium in the synthesis solution obtained by mixing the leaching solution, the fluorine absorption solution and the decomposition agent mother liquor is 6:1:(3-6).

10. A method of recovering cryolite according to claim 1, characterized in that, In Step 3: high-purity carbon can be obtained by leaching the carbon residue with a hydrochloric acid solution, and the high-purity carbon can be used to prepare electrolytic aluminum anode or cathode carbon blocks.

Images