Process method for preparing aluminum fluoride by recovering electrolytic aluminum electrolyte

Abstract

The invention discloses a process method for preparing aluminum fluoride by recovering electrolytic aluminum electrolyte, and relates to the technical field of waste material recovery. The invention relates to a process method for preparing aluminum fluoride by recovering electrolytic aluminum electrolyte. The preparation method comprises the following steps: firstly, grinding the carbon residue or electrolyte into powder, wherein the fineness is greater than 150 meshes; adding water into the powder, pulping, and adding a certain amount of aluminum sulfate or aluminum hydroxide; sulfuric acid is added for leaching under the stirring condition, the leaching temperature ranges from 70 DEG C to 100 DEG C, the leaching time ranges from 2 h to 5 h, and the pH of the leaching end point ranges from 1 to 2; adding water to dilute the filtrate, adding caustic soda flakes to adjust the pH value to 12-14 to remove impurities, and filtering out insoluble hydroxides to obtain a mixed salt solution of sodium fluoride and sodium metaaluminate; adding sulfuric acid into the mixed salt solution while stirring to adjust the pH value to 3-6, and reacting at the temperature of 60-100 DEG C for 2-5 hours to obtain aluminum fluoride slurry; and filtering and washing the slurry to obtain aluminum fluoride trihydrate, and calcining at 500-700 DEG C to obtain anhydrous aluminum fluoride with the purity of more than 95%. The method turns waste into wealth, recycles resources, and is short in process flow, low in cost and high in recovery rate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of waste material recycling, and particularly relates to a process method for recycling electrolytic aluminum electrolyte to prepare aluminum fluoride. BACKGROUND

[0002] In the process of aluminum electrolysis production, the anode carbon block of the electrolytic cell is peeled off due to high-temperature reaction and mechanical scouring, and the peeled carbon block enters the molten electrolyte (mainly composed of cryolite Na3AlF6) to form mixed waste slag, which is the waste carbon slag (referred to as "waste carbon slag" for short) of the aluminum electrolytic cell. The waste slag usually floats on the surface of the electrolyte and needs to be cleaned regularly to ensure the normal operation of the electrolytic cell, and its annual output accounts for about 3%-5% of the output of electrolytic aluminum, which is one of the typical industrial solid wastes in the electrolytic aluminum industry.

[0003] The main components of the waste carbon slag are carbonaceous substances and electrolyte, and the electrolyte has cryolite (Na3AlF6) as the core component, and the weight ratio is usually 50%-90%, and the rest is carbon particles and a small amount of impurities (such as CaO, Fe2O3, MgO and other metal oxides). Cryolite, as a key fluxing agent for aluminum electrolysis, has high added value, but the cryolite in the waste carbon slag is difficult to directly reuse due to the inclusion of carbon particles and impurities.

[0004] At present, the treatment methods for waste carbon slag in the industry have obvious limitations: 1. A small amount of waste carbon slag is reused in the electrolytic cell after simple screening, but the high carbon content easily leads to a decrease in electrolytic efficiency, and the reuse ratio is usually less than 20%; 2. Most of the waste carbon slag is used as a slagging agent for steel smelting, only using the fluxing properties of fluorides, and due to the insufficient purity, the unit value is only 1 / 5-1 / 3 of the original cryolite, and the resource utilization rate is low; 3. Some enterprises use landfill disposal, which not only occupies land resources, but also may cause soil and groundwater pollution due to the dissolution of fluorides, which does not meet the "reduction, resource utilization and harmlessness" principle in the "Solid Waste Pollution Environment Prevention Law".

[0005] In the prior art, the methods for recycling cryolite from waste carbon slag mainly include flotation separation and acid leaching. The flotation method separates carbon and cryolite by reagent, but has the problems of large reagent consumption and less than 80% of the recovery rate of cryolite; the acid leaching method can dissolve cryolite, but the impurity ions (such as Ca 2+ , Fe 3+ ) are not separated, resulting in a product purity of less than 90%, which cannot meet the quality requirements of industrial-grade aluminum fluoride. In addition, the above methods do not realize the directional conversion and efficient enrichment of aluminum and fluorine elements, which limits the economic value of resource recycling.

[0006] Therefore, developing a process method capable of efficiently separating carbon and electrolyte in waste carbon residue, and directionally preparing high-purity aluminum fluoride, which has environmental protection and economic benefits, has important significance for improving the solid waste resource level of the electrolytic aluminum industry, reducing production costs, and reducing environmental pollution. SUMMARY

[0007] The present application aims to at least solve one of the technical problems existing in the prior art, and provides a process method for recycling electrolytic aluminum electrolyte to prepare aluminum fluoride, to solve the problems raised in the background art.

[0008] To achieve the above-mentioned purpose, the present application provides the following technical solution: a process method for recycling electrolytic aluminum electrolyte to prepare aluminum fluoride, the preparation method comprising the following steps: S1. Grinding the carbon residue or electrolyte to a fineness of >150 mesh; S2. Adding water to the powder to make a slurry, and adding aluminum sulfate or aluminum hydroxide, wherein the Al:F molar ratio is (1-1.2):3; S3. Adding sulfuric acid under stirring conditions for leaching, the leaching temperature is 70-100℃, the leaching time is 2-5h, the leaching end point pH is 1-2, the amount of sulfuric acid added is 20-80% of the total mass of Na3AlF6 of the raw material, and the solid-liquid ratio of leaching is 1:(3-5); S4. Diluting the filtrate with water by 3-5 times, adding flake alkali to adjust the pH to 12-14 for impurity removal, and filtering the insoluble hydroxide (mainly calcium hydroxide, iron hydroxide, and magnesium hydroxide) to obtain a mixed salt solution of sodium fluoride and sodium metaaluminate; S5. Adding sulfuric acid to the mixed salt solution to adjust the pH to 3-6, the reaction temperature is 60-100℃, and the reaction time is 2-5h, to obtain aluminum fluoride slurry; S6. Filtering and washing the slurry to obtain aluminum fluoride trihydrate, and calcining the aluminum fluoride trihydrate at 500-700℃ to obtain anhydrous aluminum fluoride with a purity of >95%.

[0009] Preferably, the content of Na3AlF6 component in the electrolyte is >80%, and the content of Na3AlF6 component in the carbon residue is >60%.

[0010] Preferably, the carbon residue in S1 needs to be pretreated before grinding, including oil removal by flotation and drying, the amount of collector (such as kerosene) used in the flotation process is 300-1000 grams per ton of carbon residue, and the amount of frother (such as No.2 oil) used is 200-500 grams, and the carbon powder recovery rate after flotation is >80%.

[0011] Preferably, mechanical stirring or ultrasonic assisted stirring is used in the leaching process in S3, and the stirring speed is 200-400rpm to strengthen the solid-liquid mass transfer.

[0012] Preferably, the calcination process in S6 adopts gradient heating, specifically: first, heating at 5-10℃ / min to 300-400℃, holding for 1-2h, then heating at 10-15℃ / min to 500-700℃, holding for 2-3h, the oxygen content of the product after calcination is <0.5%.

[0013] Preferably, the filtrate after impurity removal in S4 is further treated to recover lithium elements, specifically including: adding sodium carbonate solution to the filtrate to adjust the pH to 8-10, precipitating and separating lithium carbonate, and the lithium extraction rate is >75%.

[0014] Preferably, after the reaction in S5 is completed, the aluminum fluoride slurry is filtered by pressure filtration or centrifugal filtration, and the filter cake is washed with deionized water until the conductivity is <100μS / cm.

[0015] Preferably, the anhydrous aluminum fluoride after calcination in S6 is crushed by airflow to 200-325 mesh, which can reduce the initial crystallization temperature of the electrolyte by 5-10℃ when used for aluminum electrolysis.

[0016] Preferably, the leaching solution after treatment in S3 is recycled, the recycling times are 2-3 times, and the fluorine recovery rate is >90%.

[0017] Preferably, the hydroxide filter residue produced by impurity removal in S4 is acid-dissolved to recover metal elements such as iron and calcium, and the metal recovery rate is >95%.

[0018] Compared with the prior art, the beneficial effects of the present application are: The process method for recycling and preparing aluminum fluoride from aluminum electrolysis electrolyte has the following advantages: waste resourceization: converting fluorine-containing hazardous waste (electrolyte / carbon residue) into high-value-added product anhydrous aluminum fluoride (AlF3), achieving a fluorine recovery rate of ≥90%, and solving the problem of solid waste storage pollution in the aluminum electrolysis industry; component adaptability: for raw materials with a Na3AlF6 content of >60%, through the synergistic leaching mechanism of sulfuric acid+aluminum salt additives, F and Al elements are effectively dissociated, HF gas pollution generated by traditional acid methods is avoided, the process flow is simplified, and the cost is optimized; reaction efficiency improvement: the solid-liquid ratio combination is used in the leaching stage, the reaction time is shortened to 2-5h, and the energy consumption is reduced by more than 30%; the aluminum salt additive is added according to Al:F=(1-1.2):3 to accurately control the ion balance, reduce the amount of sulfuric acid, and has strong impurity removal directionality; after diluting the leaching solution by 3-5 times, the pH is adjusted to 12-14, Ca 2+ , Fe 3+ , Mg 2+ (removal rate >98%) is selectively precipitated, avoiding the introduction of new impurities; product quality and economic benefits are outstanding; high-purity product: through pH 3-6 directional synthesis of aluminum fluoride and calcination, anhydrous AlF3 with a purity of >96% is obtained, meeting the industrial first-grade product standard; cost reduction: the raw material is waste electrolyte, saving the cost of mineral mining; the process does not have high-temperature melting or complex purification steps, and the comprehensive production cost is reduced by more than 40% compared with the traditional process. DETAILED DESCRIPTION

[0019] This part will describe the specific embodiments of the present application in detail, so that people can intuitively and visually understand each technical feature and the overall technical solution of the present application, but it cannot be understood as a limitation on the protection scope of the present application.

[0020] The present application provides a technical solution: a process method for recovering and preparing aluminum fluoride from electrolytic aluminum electrolyte, and the implementation conditions are as follows: Raw material requirements: The content of Na3AlF6 in the electrolytic aluminum electrolyte is ≥80%, and the content of Na3AlF6 in the carbon residue is ≥60%.

[0021] The aluminum salt additive can be aluminum sulfate (Al2(SO4)3) or aluminum hydroxide (Al(OH)3), and the addition amount is controlled as (1-1.2):3 according to the molar ratio of total F and Al in the raw material.

[0022] The addition amount of sulfuric acid is 20%-80% of the total mass of Na3AlF6 in the raw material.

[0023] Key process parameters: The solid-liquid ratio of the leaching step is 1:(3-5), the temperature is 70-100℃, the time is 2-5h, and the end point pH is 1-2.

[0024] The dilution multiple of the impurity removal step is 3-5 times, the pH is 12-14, and the generated hydroxide precipitate is mainly Ca(OH)2, Fe(OH)3, and Mg(OH)2.

[0025] The pH of the aluminum fluoride synthesis step is 3-6, the temperature is 60-100℃, and the time is 2-5h.

[0026] The calcination temperature is 500-700℃, and the purity of the product anhydrous aluminum fluoride is ≥95%.

[0027] Example 1 (carbon residue as raw material) Raw material treatment: Grind the carbon residue containing 65% Na3AlF6 to a particle size of >150 mesh, and weigh 1000kg.

[0028] Leaching reaction: Add 3000L water to the reaction kettle for slurry, and add 185kg of aluminum hydroxide (Al:F molar ratio=1.05:3).

[0029] Slowly add 500kg of sulfuric acid (77% of the mass of Na3AlF6) under stirring, and heat to 95℃ for 4h (solid-liquid ratio 1:3).

[0030] Filter the leaching solution (pH=1.2, F - 9.5%, Al3+ 5.2%.

[0031] Purification; The leachate was diluted 4 times with water, sodium hydroxide was added to adjust the pH to 13, and the precipitates such as Ca(OH)2 and Fe(OH)3 were removed by filtration.

[0032] The filtrate contains F - 2.1%, Al 3+ 1.2%.

[0033] Synthetic aluminum fluoride: Sulfuric acid was added to the filtrate to adjust the pH to 4, and the mixture was stirred at 90°C for 4 hours to produce aluminum fluoride slurry.

[0034] Post-processing: The slurry was filtered, washed, and dried to obtain aluminum fluoride trihydrate (AlF3·3H2O).

[0035] Anhydrous aluminum fluoride was obtained by calcination at 700℃ for 4 hours, and the purity was 96.4% as determined by X-ray fluorescence spectroscopy.

[0036] Example 2 (electrolyte as raw material, aluminum hydroxide as additive) Raw material processing: Grind the electrolyte containing 685% Na3AlF to a particle size >150 mesh, and weigh 1000 kg.

[0037] Leaching reaction: Add 4000L of water to make a slurry (solid-liquid ratio 1:4), and add 250kg of aluminum hydroxide (Al:F molar ratio = 1.1:3).

[0038] Add 700 kg of sulfuric acid (82% of the mass of Na3AlF6) and react at 95°C for 4 hours.

[0039] The leachate obtained by filtration (pH=1.0, containing F) - 8.7%, Al 3+ 4.5%.

[0040] Impurity removal and synthesis: The leachate was diluted 4 times, and after adding NaOH to adjust the pH to 13, it was filtered.

[0041] The filtrate was adjusted to pH 5 and reacted at 95℃ for 4 hours to obtain aluminum fluoride slurry.

[0042] Calcination: The washed and dried aluminum fluoride trihydrate was calcined at 650℃ for 4 hours, and the product purity was 97.4%.

[0043] Example 3 (electrolyte as raw material, aluminum sulfate as additive) Raw material processing: Same as in Example 2, take 1000 kg of electrolyte powder (Na3AlF685%).

[0044] Leaching reaction: Add 4000L of water to make a pulp, and add 650kg of aluminum sulfate (Al:F molar ratio = 1.2:3).

[0045] Add 150 kg of sulfuric acid (18% of the mass of Na3AlF6) and react at 95°C for 4 h.

[0046] The leachate obtained by filtration (pH=1.3, containing F) - 8.1%, Al 3+ 4.2%.

[0047] Impurity removal and synthesis: The leachate was diluted 3 times, and after adding NaOH to adjust the pH to 13, it was filtered.

[0048] The filtrate was adjusted to pH 4.5 and reacted at 95℃ for 4 hours to obtain a slurry.

[0049] Calcination: The purity of anhydrous aluminum fluoride after calcination at 650℃ for 4 hours was 97.6%.

[0050] Technical effect The above examples demonstrate that: The use of sulfuric acid and aluminum salt auxiliaries for synergistic dissolution significantly improves the leaching rate of fluorine and aluminum (>95%).

[0051] Stepwise pH control achieves efficient impurity removal (Ca) 2+ Fe 3+ Mg 2+ (Removal rate > 98%) and directional synthesis of aluminum fluoride.

[0052] Using waste electrolytes or carbon slag as raw materials, the aluminum fluoride recovery rate is ≥90%, the product purity is >96%, and it meets industrial standards.

[0053] The invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the invention.

Claims

1. A process for the recovery of aluminum fluoride from an electrolytic aluminum electrolyte, characterized in that, The preparation method comprises the following steps: S1. Grinding the carbon residue or electrolyte to a fineness of >150 mesh; S2. Adding water to the powder to make a pulp, and adding aluminum sulfate or aluminum hydroxide, wherein the molar ratio of Al:F is (1-1.2):3; S3. Adding sulfuric acid under stirring to leach, the leaching temperature is 70-100℃, the leaching time is 2-5h, the leaching end point pH is 1-2, the amount of sulfuric acid added is 20-80% of the total mass of Na3AlF6 of the raw material, and the solid-liquid ratio of leaching is 1:(3-5); S4. Diluting the filtrate with water by 3-5 times, adding flake caustic soda to adjust the pH to 12-14 to remove impurities, filtering the insoluble hydroxide to obtain a mixed salt solution of sodium fluoride and sodium metaaluminate; S5. Adding sulfuric acid to the mixed salt solution to adjust the pH to 3-6, the reaction temperature is 60-100℃, the reaction time is 2-5h, to obtain aluminum fluoride slurry; S6. Filtering and washing the slurry to obtain aluminum fluoride trihydrate, and calcining the aluminum fluoride trihydrate at 500-700℃ to obtain anhydrous aluminum fluoride with a purity of >95%.

2. A process for the recovery of aluminium fluoride from spent electrolytic aluminium electrolyte according to claim 1, characterised in that, The content of Na3AlF6 component in the electrolyte is >80%, and the content of Na3AlF6 component in the carbon residue is >60%.

3. A process for the recovery of aluminium fluoride from spent electrolytic aluminium electrolyte according to claim 1, characterised by the fact that, The carbon residue in S1 needs to be pretreated before grinding, including oil removal by flotation and drying, the amount of collector (such as kerosene) used in the flotation process is 300-1000 grams per ton of carbon residue, and the amount of frother (such as No. 2 oil) used is 200-500 grams, the carbon powder recovery rate after flotation is >80%.

4. A process for the recovery of aluminium fluoride from spent electrolytic aluminium electrolyte according to claim 3, characterised by the fact that, In S3, mechanical stirring or ultrasonic assisted stirring is used in the leaching process, and the stirring speed is 200-400 rpm to strengthen the solid-liquid mass transfer.

5. A process for the recovery of aluminium fluoride from spent electrolytic aluminium electrolyte according to claim 1, characterised by the fact that, In S6, the calcination process adopts gradient heating, specifically: first, increase the temperature to 300-400℃ at a rate of 5-10℃ / min, and keep the temperature for 1-2h, then increase the temperature to 500-700℃ at a rate of 10-15℃ / min, and keep the temperature for 2-3h, the oxygen content of the product after calcination is <0.5%.

6. A process for the recovery of aluminium fluoride from spent electrolytic aluminium electrolyte according to claim 1, characterised by the fact that, In S4, the filtrate after impurity removal is further treated to recover lithium elements, specifically including: adding sodium carbonate solution to the filtrate to adjust the pH to 8-10, precipitating and separating lithium carbonate, and the lithium extraction rate is >75%.

7. A process for the recovery of aluminium fluoride from spent electrolytic aluminium electrolyte according to claim 4, characterised by the fact that, After the reaction in S5, the aluminum fluoride slurry is filtered by pressure filtration or centrifugal filtration, and the filter cake is washed with deionized water until the conductivity is <100μS / cm.

8. A process for the recovery of aluminium fluoride from spent electrolytic aluminium electrolyte according to claim 6, characterised by the fact that, After calcination in S6, the anhydrous aluminum fluoride is ground by airflow to 200-325 mesh, which can reduce the initial crystallization temperature of the electrolyte by 5-10℃ when used for aluminum electrolysis.

9. The process of claim 1, wherein, The leaching liquid in S3 is recycled after treatment, and the recycling number is 2-3 times, and the fluorine recovery rate is >90%.

10. The process of claim 1, wherein, The hydroxide filter residue produced in S4 is acid-dissolved to recover iron, calcium and other metal elements, and the metal recovery rate is >95%.