Method for recycling waste liquid from purification of carbonaceous product of hazardous waste vacuum distillation in aluminum electrolysis

Abstract

The method for recycling waste liquid of aluminum electrolysis hazardous waste vacuum distillation carbonaceous product purification belongs to the field of metallurgy and environmental technology. First, excessive NaCl is added to the fluorine-containing waste liquid, and the pH is adjusted to make Na + and [AlF6] 3－ in the waste liquid form cryolite and precipitate, after filtration, sodium hydroxide solution is added to the filtrate to adjust the pH value, so that Ca 2+ forms Ca(OH)2 precipitate, and Al 3+ forms Al(OH)3 precipitate first, and is converted into NaAlO2 and returns to the solution with the increase of the pH of the solution, a small amount of hydrochloric acid is added to the filtrate after filtration to adjust the pH of the filtrate to 8-10, so that NaAlO2 changes back to Al(OH)3 precipitate, then excessive concentrated hydrochloric acid is added to the precipitate to obtain AlCl3 solution, which is reused in the purification process of the distillation carbon block, and a small amount of concentrated hydrochloric acid is added to the filtrate to adjust the pH to neutral, and then evaporation crystallization is carried out to obtain NaCl crystals, which are reused in the preparation of cryolite, and the harmless treatment and full-component recycling of the waste liquid of the distillation carbon block purification are completed.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy and environmental technology, and specifically relates to a method for the full-component resource recovery and utilization of fluorine-containing strongly acidic waste liquid. Background Technology

[0002] Cryolite-alumina molten salt electrolysis is currently the world's most widely used industrial process for primary aluminum production, with the aluminum electrolysis cell as its main equipment. During primary aluminum production, the lining of the aluminum electrolysis cell is corroded by electrolytes, alkali metals, and other substances. Some of these substances can also react with the lining material, causing it to fail or break. Therefore, after long-term operation, aluminum electrolysis cells must be shut down for major overhauls. This process generates a large amount of waste aluminum electrolysis cell lining, also known as overhaul slag. The waste lining can be mainly divided into two parts: carbonaceous waste cathode carbon blocks and non-carbonaceous waste refractory materials. The leaching toxicity of soluble fluorides (NaF, Na3AlF6, etc.) and cyanides in these materials far exceeds national standards. Directly dumping them without treatment will seriously pollute the environment and threaten human life and health. Therefore, the new version of the "National Hazardous Waste List" implemented in 2016 lists waste aluminum electrolysis cell lining as hazardous solid waste, with the code 321-023-48. However, the linings of waste aluminum electrolysis cells contain substances such as carbon blocks, fluorides, and alkali metals, all of which have significant recycling value. Therefore, how to safely dispose of and recycle these waste linings has always been a hot research topic in the aluminum industry. Currently, the main methods for the comprehensive utilization of waste aluminum electrolysis cell linings include:

[0003] CN116715263A discloses a method for treating and recycling waste tank linings of aluminum electrolytic cells. The method first mixes the waste tank linings with concentrated sulfuric acid, and then volatilizes hydrogen fluoride and sulfur trioxide stepwise at high temperature. After collection, cryolite and sulfuric acid are produced, while aluminum salts are transformed into stable corundum sand. After water leaching, a lithium-containing aqueous solution is obtained. Carbon dioxide is then introduced to obtain lithium carbonate. The remaining liquid is recycled as a leachate until saturated sodium sulfate crystals precipitate, yielding sodium sulfate product.

[0004] CN116553591A discloses a process for recovering fluoride salts from the lining of waste cells in aluminum electrolysis cells. The process involves dividing the overhaul slag into two categories, flotating the high-carbon powder, and then leaching and hydrolyzing both types of powder to precipitate soluble fluoride salts from the waste. The resulting high-alumina silicon powder can be used as a raw material for cement, bricks, etc. The processed carbon powder can be used as fuel, the generated sodium fluoride can be used as a substitute for fluorite, and the industrial salt obtained from crystallization can also be sold.

[0005] CN116715262A discloses a method for the resource recycling of waste tank linings. The method first adds calcium chloride and hydrogen peroxide to the waste tank lining, and after pressurization, obtains a lithium-containing solution and a leaching residue containing calcium fluoride. Then, the leaching residue is subjected to stepwise flotation to obtain carbon concentrate, calcium fluoride and cryolite. The leachate is then purified, concentrated and hydrothermally prepared into a high-value lithium carbonate product.

[0006] US4444740A discloses a method for recovering fluoride salts from the lining of an aluminum electrolysis waste cell. The waste cell lining is crushed and calcined, then the residue is leached with dilute alkali, filtered, and calcium salts are added to the filtrate to produce calcium fluoride. Concentrated sulfuric acid is then added to obtain hydrogen fluoride and calcium sulfate. The hydrogen fluoride is passed through an alumina dry scrubber to obtain alumina containing fluorides, which is used as a raw material for aluminum electrolysis production. The calcium sulfate, after lime treatment, can be disposed of as landfill material.

[0007] The above-mentioned methods for the comprehensive utilization of aluminum electrolysis waste cell linings have drawbacks such as low product purity, inability to achieve full component recovery, and severe equipment corrosion. To date, the industry has not found an effective method to achieve safe separation and full component resource utilization of aluminum electrolysis solid waste. Because the saturated vapor pressures of fluorides (sodium fluoride, sodium fluoroaluminate, lithium fluoride, etc.) and alkali metals in aluminum electrolysis waste cell linings are high, vacuum distillation can be used to treat the waste cell linings, separating the fluorides and alkali metals in gaseous form for condensation and collection. Building upon this foundation, the research team led by Feng Naixiang at Northeastern University proposed a method and related equipment for vacuum distillation treatment of waste aluminum electrolytic cell linings. They also disclosed a series of patents, such as CN104894600A, which discloses a method for separating and recovering carbon and electrolyte components from carbon-containing solid waste from aluminum molten salt electrolysis, and CN218980477U, which discloses a vacuum distillation fluoride electrolyte collection device for industrial aluminum electrolytic cell overhaul slag. These methods and devices enable the complete recycling of waste cell lining components. This process effectively solves problems existing in aluminum electrolytic cell waste cell lining recycling technologies, such as incomplete decomposition of toxic components, incomplete recovery of valuable components, and ineffective recycling of waste refractory materials. Specifically, by vacuum distilling waste cathode carbon blocks from aluminum electrolytic cells at 1000-1200℃, fluorides and alkali metals can be effectively separated and recovered. Cyanides can also be completely decomposed at high temperatures. After vacuum distillation treatment, the waste aluminum electrolytic cell lining yields carbonaceous materials, fluoride electrolytes, metallic sodium, and refractory materials, achieving complete component utilization. The carbon blocks obtained from vacuum distillation were tested and found to contain over 94% carbon (C). Phase analysis revealed that the main impurity was calcium fluoride. This is because the saturated vapor pressure of calcium fluoride in the waste cathode carbon blocks is low, making it impossible to distill off under vacuum conditions at 1200℃. Furthermore, due to the low saturated vapor pressure of Al... 3+ With F － It can form stable complex ions [AlF6]. 3－Therefore, CN109437149A proposes a method for purifying waste cathode carbon blocks from aluminum electrolysis cells. This method involves dissolving residual calcium fluoride impurities in the distilled carbon blocks using aluminum chloride solution, further purifying the carbon blocks. The resulting purified carbon blocks have a carbon content exceeding 98%, thus further increasing their recycling value. However, this method generates purification waste liquid, the main component of which is Al. 3+ Ca 2+ Cl - [AlF6] 3－ The pH value is 1-2.5. Direct discharge of this waste liquid will not only re-pollute the environment but also waste resources. Summary of the Invention

[0008] To address the significant environmental pollution caused by the highly acidic waste liquid generated during the purification of aluminum chloride solution from distillation charcoal blocks, and the lack of recovery of fluoride salts, this invention provides a method for the resource-based recycling of fluoride-containing waste liquid. First, excess NaCl is added to the fluoride-containing waste liquid, and the pH is adjusted to ensure that the NaCl content is within acceptable limits. + [AlF6] in waste liquid 3－ Cryolite forms and precipitates. After filtration, sodium hydroxide solution is added to the filtrate to adjust the pH to 11.7-14, so that the Ca in the solution... 2+ Ca(OH)₂ precipitate is formed, while Al 3+ First, Al(OH)3 precipitate forms. As the pH of the solution increases, it transforms into NaAlO2 and returns to the solution. After filtration, a small amount of hydrochloric acid is added to the filtrate to adjust the pH to 8-10, causing NaAlO2 to revert back to Al(OH)3 precipitate. Then, excess concentrated hydrochloric acid is added to the precipitate to obtain AlCl3 solution, which is reused in the purification process of distilled charcoal blocks. At the same time, a small amount of concentrated hydrochloric acid is added to the filtrate to adjust the pH to neutral, and then it is evaporated and crystallized to obtain NaCl crystals, which are reused to prepare cryolite. This completes the harmless treatment and recycling of the waste liquid from the purification of distilled charcoal blocks.

[0009] A method for the resource recovery and utilization of waste liquid from vacuum distillation of carbonaceous products from spent aluminum electrolysis cathode carbon blocks, such as... Figure 1 As shown, it includes the following steps:

[0010] (1) Preparation of cryolite

[0011] After enriching the purified waste liquid, it is heated to 100℃ and concentrated to achieve a sulfur (F) content of 0.5-5 mol / L. Then, solid NaCl is added at 80-99℃ with a liquid-to-solid ratio of 2-5:1 to provide Na+ to the waste liquid. +The pH of the waste liquid was then adjusted to 3-4, and after being kept at 80-99℃ for 0.5-4 hours, it was filtered to obtain filtrate A1 and filter residue B1. Filter residue B1 was washed and dried to obtain cryolite crystals. Reaction formula (1) represents the reaction that occurred during this process.

[0012]

[0013] (2) Separation of Al 3+ and Ca 2+

[0014] Sodium hydroxide solution was added to the filtrate A1 obtained in step (1). The filtrate first underwent reactions (2) and (3), resulting in a white flocculent precipitate, the main components of which were Al(OH)3 and Ca(OH)2. As sodium hydroxide solution was added until the pH of the solution was 11-14, the Al(OH)3 precipitate dissolved, generating NaAlO2. Sodium hydroxide solution was added dropwise until the precipitate no longer decreased. The filtrate A2 and filter residue B2 were obtained by filtration. Filter residue B2 was washed with deionized water and dried to obtain Ca(OH)2 precipitate. Reaction (4) is the reaction that occurs when the precipitate dissolves.

[0015] AlCl3+3NaOH=3NaCl+Al(OH)3↓ (2)

[0016] CaCl2+2NaOH=2NaCl+Ca(OH)2↓ (3)

[0017] Al(OH)3+NaOH=NaAlO2+2H2O (4)

[0018] (3) Filtrate treatment

[0019] Add concentrated hydrochloric acid dropwise to the filtrate A2 obtained in step (2), with an acid-to-liquid ratio of 10-40:1, to lower the pH of the solution to 8-10. NaAlO2 in the solution is converted into Al(OH)3, generating a large amount of white flocculent precipitate. After observing that the precipitate no longer continues to form, filter to obtain filtrate A3 and filter residue B3. Wash filter residue B3 to obtain Al(OH)3 precipitate. Add concentrated hydrochloric acid to filtrate A3 to adjust the pH of the solution to neutral. Then place it in a resistance furnace and heat it at 80-120℃ to evaporate and crystallize it to obtain NaCl crystals, which are reused in the preparation of cryolite in step (1).

[0020] The Al(OH)3 obtained in step (3) can be processed in two ways:

[0021] 1) Calcining Al(OH)3 to produce Al2O3, which is then returned to the aluminum electrolysis cell as a raw material; or

[0022] 2) Add concentrated hydrochloric acid to the Al(OH)3 precipitate to obtain aluminum chloride solution, which is then reused for the purification of distilled carbon blocks.

[0023] In step (1), the obtained cryolite has a purity of over 97%.

[0024] In step (2), the concentration of the added sodium hydroxide solution is 2-5 mol / L.

[0025] In step (3), the added concentrated hydrochloric acid contains 36.0-38.0 wt% hydrochloric acid, and the remaining impurities include sulfite, tin, lead, arsenic, copper and iron, all of which contain less than 0.001%.

[0026] In step (3), the resistance furnace used is a crucible resistance furnace or a rotary kiln, which uses electric heating or water gas or natural gas as a heat source.

[0027] The beneficial effects of this invention are:

[0028] The fluorine-containing waste liquid recycling method of this invention can achieve the harmless treatment and full-component recycling of the strongly acidic fluorine-containing waste liquid generated during the purification of aluminum chloride, a product of aluminum electrolysis cell cathode distillation. The products are cryolite, calcium hydroxide, aluminum hydroxide, and sodium chloride. There is no discharge of wastewater, waste gas, or waste residue, and no secondary pollution is generated. Sodium chloride can be added back to the fluorine-containing waste liquid to prepare cryolite, and aluminum hydroxide can be calcined into alumina for use as a raw material for aluminum electrolysis, or prepared into aluminum chloride for reuse in the purification of distillation carbon blocks. It is an environmentally friendly treatment method with significant effects and good application prospects. Attached Figure Description

[0029] Figure 1 This is a process flow diagram of the present invention;

[0030] Figure 2 The image shows the X-ray diffraction phase analysis diagram of the cryolite product obtained in step 1 of Example 1. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to the embodiments.

[0032] Example 1

[0033] (1) Example 1 involved purifying carbon block waste liquid from 2000 ml of aluminum chloride solution. The solution was evaporated and concentrated to 200 ml, with a total fluorine concentration of 1.4 mol / L. 60 g of sodium chloride solid was added, along with an appropriate amount of sodium hydroxide solution. The pH was adjusted to 3.5, and the solution was kept at 80°C for 2 hours. A white precipitate appeared. After filtration, the filter residue was washed and dried, yielding 8.57 g of precipitate, containing 97% cryolite and a small amount of CaF2 as the main impurity. X-ray diffraction phase analysis of the precipitate is as follows: Figure 2 As shown.

[0034] (2) Add 2 mol / L sodium hydroxide solution to the filtrate obtained in step (1) and adjust the pH of the filtrate to 12.0. It can be observed that a large amount of white flocculent precipitate first appears in the filtrate. As the pH of the filtrate increases, some of the precipitate dissolves. Continue to add sodium hydroxide solution until the precipitate no longer decreases. Filter, wash the filter residue with a small amount of deionized water, and dry to obtain 10.5g of Ca(OH)2 with a purity of more than 96%.

[0035] (3) Add a small amount of concentrated hydrochloric acid to the filtrate obtained in step (2) to adjust the pH to 9.5. A large amount of white flocculent precipitate appears in the solution. After filtration, add excess concentrated hydrochloric acid to the filter residue until the precipitate is completely dissolved to obtain aluminum chloride solution. Finally, adjust the pH of the filtrate to neutral and place it in an electric resistance furnace to evaporate and crystallize to obtain sodium chloride crystals.

[0036] Example 2

[0037] (1) Example 2 was to process 4000ml aluminum chloride solution to purify carbon block experimental waste liquid. The solution was evaporated and concentrated to 200ml, with a total fluorine concentration of 3mol / L. 90g of sodium chloride solid was added to it, and an appropriate amount of sodium hydroxide solution was added to adjust the pH of the solution to 3.0. The solution was kept at 80℃ for 2h, and a white precipitate appeared. After filtration, the filter residue was washed and dried to obtain 11.27g of precipitate, of which the content of cryolite was 98%, and the impurities were mainly a small amount of CaF2.

[0038] (2) Add 2 mol / L sodium hydroxide solution to the filtrate obtained in step (1) and adjust the pH of the filtrate to 13.0. It can be observed that a large amount of white flocculent precipitate first appears in the filtrate. As the pH of the filtrate increases, some of the precipitate dissolves. Continue to add sodium hydroxide solution until the precipitate no longer decreases. Filter, wash the filter residue with a small amount of deionized water, and dry to obtain 15.5g of Ca(OH)2 with a purity of 98%.

[0039] (3) Add a small amount of concentrated hydrochloric acid to the filtrate obtained in step (2) to adjust the pH to 9.0. A large amount of white flocculent precipitate appears in the solution. After filtration, add excess concentrated hydrochloric acid to the filter residue until the precipitate is completely dissolved to obtain aluminum chloride solution. Finally, adjust the pH of the filtrate to neutral and place it in an electric resistance furnace to evaporate and crystallize to obtain sodium chloride crystals.

[0040] Example 3

[0041] (1) Example 3 is an aqueous solution obtained by dissolving CaF2 in 200 ml of aluminum chloride solution. The concentration of AlCl3 in the aluminum chloride solution is 200 g / L, the mass of CaF2 is 10 g, and the total concentration of fluorine in the solution is 1.5 mol / L. 55 g of sodium chloride solid is added to the solution, and an appropriate amount of sodium hydroxide solution is added to adjust the pH of the solution to 3.7. The solution is kept at 80°C for 2 h. A white precipitate appears in the solution. After filtration, the filter residue is washed and dried to obtain 9.23 g of precipitate, of which the content of cryolite is 97%, and the impurities are mainly a small amount of CaF2.

[0042] (2) Add 2 mol / L sodium hydroxide solution to the filtrate obtained in step (1) and adjust the pH of the filtrate to 13.0. It can be observed that a large amount of white flocculent precipitate first appears in the filtrate. As the pH of the filtrate increases, some of the precipitate dissolves. Continue to add sodium hydroxide solution until the precipitate no longer decreases. Filter, wash the filter residue with a small amount of deionized water, and dry to obtain 11.2g of Ca(OH)2 with a purity of 96%.

[0043] (3) Add a small amount of concentrated hydrochloric acid to the filtrate obtained in step (2) to adjust the pH to 9.5. A large amount of white flocculent precipitate appears in the solution. After filtration, add excess concentrated hydrochloric acid to the filter residue until the precipitate is completely dissolved to obtain aluminum chloride solution. Finally, adjust the pH of the filtrate to neutral and place it in an electric resistance furnace to evaporate and crystallize to obtain sodium chloride crystals.

[0044] Example 4

[0045] (1) Example 4 is an aqueous solution obtained by dissolving CaF2 in 200 ml of aluminum chloride solution. The concentration of AlCl3 in the aluminum chloride solution is 200 g / L, the mass of CaF2 is 15 g, and the total concentration of fluorine in the solution is 2 mol / L. 65 g of sodium chloride solid is added to the solution, and an appropriate amount of sodium hydroxide solution is added to adjust the pH of the solution to 3.5. The solution is kept at 80°C for 2 h. A white precipitate appears in the solution. After filtration, the filter residue is washed and dried to obtain 12.53 g of precipitate, of which the content of cryolite is 98%, and the impurities are mainly a small amount of CaF2.

[0046] (2) Add 2 mol / L sodium hydroxide solution to the filtrate obtained in step (1) and adjust the pH of the filtrate to 13.5. It can be observed that a large amount of white flocculent precipitate first appears in the filtrate. As the pH of the filtrate increases, some of the precipitate dissolves. Continue to add sodium hydroxide solution until the precipitate no longer decreases. Filter, wash the filter residue with a small amount of deionized water, and dry to obtain 14.6g of Ca(OH)2 with a purity of 98%.

[0047] (3) Add a small amount of concentrated hydrochloric acid to the filtrate obtained in step (2) to adjust the pH to 9.0. A large amount of white flocculent precipitate appears in the solution. After filtration, add excess concentrated hydrochloric acid to the filter residue until the precipitate is completely dissolved to obtain aluminum chloride solution. Finally, adjust the pH of the filtrate to neutral and place it in an electric resistance furnace to evaporate and crystallize to obtain sodium chloride crystals.

[0048] Example 5

[0049] (1) Example 5 was to process 3000ml aluminum chloride solution to purify carbon block experimental waste liquid. The solution was evaporated and concentrated to 200ml, with a total fluorine concentration of 5mol / L. 80g of sodium chloride solid was added, and an appropriate amount of sodium hydroxide solution was added to adjust the pH of the solution to 4. The solution was kept at 85℃ for 3h. A white precipitate appeared in the solution. After filtration, the filter residue was washed and dried to obtain 10.36g of precipitate, of which the content of cryolite was 99%, and the impurities were mainly a small amount of CaF2.

[0050] (2) Add 2 mol / L sodium hydroxide solution to the filtrate obtained in step (1) and adjust the pH of the filtrate to 13.5. It can be observed that a large amount of white flocculent precipitate first appears in the filtrate. As the pH of the filtrate increases, some of the precipitate dissolves. Continue to add sodium hydroxide solution until the precipitate no longer decreases. Filter, wash the filter residue with a small amount of deionized water, and dry to obtain 14.3g of Ca(OH)2 with a purity of 98%.

[0051] (3) Add a small amount of concentrated hydrochloric acid to the filtrate obtained in step (2) to adjust the pH to 9.0. A large amount of white flocculent precipitate appears in the solution. After filtration, add excess concentrated hydrochloric acid to the filter residue until the precipitate is completely dissolved to obtain aluminum chloride solution. Finally, adjust the pH of the filtrate to neutral and place it in an electric resistance furnace to evaporate and crystallize to obtain sodium chloride crystals.

Claims

1. A method for the resource utilization of waste liquid purified from carbonaceous products of vacuum distillation in aluminum electrolysis hazardous waste, comprising the following steps: (1) Preparation of cryolite After enriching the purified waste liquid, it is heated to 100℃ and concentrated to achieve a sulfur (F) content of 0.5-5 mol / L. Then, solid NaCl is added at 80-99℃ with a liquid-to-solid ratio of 2-5:1 to provide Na+ to the waste liquid. + The pH of the waste liquid was then adjusted to 3-4, and after being kept at 80-99℃, it was filtered to obtain filtrate A1 and filter residue B1. Filter residue B1 was washed and dried to obtain cryolite crystals. The purified waste liquid refers to the strongly acidic waste liquid generated from the purification of distilled carbon blocks with aluminum chloride solution, and its main component is Al. 3+ Ca 2 + Cl - [AlF6] 3- The pH value is 1-2.5; (2) Separation of Al 3+ and Ca 2+ Add sodium hydroxide solution to the filtrate A1 obtained in step (1), and a large amount of white flocculent precipitate appears. Its main components are Al(OH)3 and Ca(OH)2. As sodium hydroxide solution is added until the pH of the solution is 11-14, the Al(OH)3 precipitate dissolves and generates NaAlO2. Continue to add sodium hydroxide solution until the precipitate no longer decreases. Filter to obtain filtrate A2 and filter residue B2. Wash filter residue B2 with deionized water and dry to obtain Ca(OH)2 precipitate. (3) Filtrate treatment Add concentrated hydrochloric acid dropwise to the filtrate A2 obtained in step (2) to lower the pH value of the solution to 8-10. NaAlO2 in the solution is converted into Al(OH)3, generating a large amount of white flocculent precipitate. After observing that the precipitate no longer continues to form, filter to obtain filtrate A3 and filter residue B3. Wash filter residue B3 to obtain Al(OH)3 precipitate. The obtained Al(OH)3 can be processed in two ways: 1) Calcine Al(OH)3 to generate Al2O3 and return it to the aluminum electrolysis cell as raw material; 2) Add concentrated hydrochloric acid to Al(OH)3 precipitate to obtain aluminum chloride solution, which can be reused for the purification of distilled carbon blocks. Add concentrated hydrochloric acid to filtrate A3 to adjust the pH value of the solution to neutral. Then heat and evaporate at 80-120℃ to obtain NaCl crystals, which can be reused for the preparation of cryolite in step (1).

2. The method for resource utilization of waste liquid purified from vacuum distillation carbonaceous products of aluminum electrolysis hazardous waste according to claim 1, characterized in that, In step (1), the obtained cryolite has a purity of over 97%.

3. The method for resource utilization of waste liquid purified from vacuum distillation carbonaceous products of aluminum electrolysis hazardous waste according to claim 1, characterized in that, In step (2), the concentration of the added sodium hydroxide solution is 2-5 mol / L.

4. The method for resource utilization of waste liquid purified from vacuum distillation carbonaceous products of aluminum electrolysis hazardous waste according to claim 1, characterized in that, In step (3), concentrated hydrochloric acid is added dropwise to filtrate A2, with an acid-to-liquid ratio of 10-40:

1.

5. The method for resource utilization of waste liquid purified from vacuum distillation carbonaceous products of aluminum electrolysis hazardous waste according to claim 1, characterized in that, In step (3), the resistance furnace used is a crucible resistance furnace or a rotary kiln, which uses electric heating or water gas or natural gas as a heat source.

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