A pollution-free treatment and recycling method for electrolytic aluminum overhaul slag

Abstract

The present invention discloses a pollution-free method for treating and recycling electrolytic aluminum overhaul slag. The method involves mixing the overhaul slag with concentrated sulfuric acid, then using a high-temperature, step-by-step volatilization mechanism of hydrogen fluoride and sulfur trioxide to produce cryolite and recycle the sulfuric acid. Under high-temperature conditions, aluminum compounds are converted into stable corundum sand, which is then separated through water leaching to obtain a lithium-containing aqueous solution. This solution is then conditioned with carbon dioxide to produce a lithium carbonate product. The remaining liquid is recycled as a leachate until saturated sodium sulfate crystals precipitate, yielding a sodium sulfate product. This method effectively achieves resourceful utilization of all components of the overhaul slag, features a simple, pollution-free production process, and high economic benefits.

Description

Technical Field

[0001] The present invention belongs to the field of hazardous solid waste resource recycling and utilization, and specifically relates to a method for non-polluting treatment and recycling of waste slag generated by carbon block peeling or damage in electrolytic cells during electrolytic aluminum production. Background Art

[0002] The rapid development of the aluminum industry has been accompanied by the generation of an increasing amount of hazardous waste. During the electrolytic aluminum production process, waste slag, often referred to as overhaul slag, is produced from the flaking or damage of carbon blocks caused by the corrosion of the electrolyte and the erosion of the high-temperature current in the electrolytic cell. It has been included in the National List of Hazardous Wastes. Annual production of overhaul slag is approximately 800,000 tons, primarily composed of carbon, fluoride, and small amounts of oxides such as aluminum, silicon, and lithium. Untreated storage or landfilling not only severely harms the environment but also wastes the valuable materials it contains. Currently, existing overhaul slag disposal processes include flotation, chemical leaching, and co-processing. All of these methods can harmlessly dispose of overhaul slag or recover the valuable materials it contains, but their primary focus is on harmlessly disposing of it or recovering valuable elements such as carbon, aluminum, and fluorine.

[0003] Application No. 202210517640.4 invented a method for harmless disposal and resource utilization of electrolytic aluminum overhaul slag. The method mainly involves mixing the overhaul slag with an oxidant and a catalyst, then wet-grinding it to render the cyanide harmless under oxygen conditions. The cyanide product is then defluorinated under concentrated sulfuric acid heat treatment to produce a calcium fluoride product. The remaining residue is washed with an alkaline solution and water to obtain a general solid waste residue. Although this method can harmlessly dispose of the overhaul slag and recycle the fluorine element it contains, it fails to consider the subsequent disposal of large amounts of water washing liquid (secondary hazardous waste) and the recycling of other valuable materials, failing to achieve maximum economic benefits. Application No. 202110934156.7 invented a method for the long-term stabilization and resource utilization of overhaul slag waste refractory materials. The method includes the steps of primary reaction, conversion reaction, secondary reaction, recovery of aluminum hydroxide, spontaneous reaction, alkali-excited reaction, phase reconstruction and curing and solidification. The overhaul slag is finally converted into a four-layer coating of calcium fluoride, calcium sulfate or calcium carbonate, calcium silicate and geopolymer cementitious material, thus avoiding the harm of fluorine to the environment and realizing the resource recovery of overhaul slag. However, the process of this method is relatively cumbersome, the product is of general value, and the economic benefits are low.

[0004] In short, the current disposal methods for overhaul slag still cannot recycle high-value materials and may produce secondary hazardous waste. Summary of the Invention

[0005] In response to the above-mentioned problems existing in the prior art, the present invention provides a pollution-free treatment and recycling method for electrolytic aluminum overhaul slag, aiming to fully recover high-value substances in the overhaul slag and dispose of the overhaul slag harmlessly.

[0006] The present invention provides a pollution-free treatment and recycling method for electrolytic aluminum overhaul slag, comprising the following steps:

[0007] 1) crushing and screening the overhaul slag to obtain pretreated overhaul slag;

[0008] 2) uniformly mixing the pretreated overhaul slag obtained in step 1) with a set amount of concentrated sulfuric acid to obtain a mixed material;

[0009] 3) placing the mixed material obtained in step 2) in a tubular furnace for roasting, and obtaining a roasted material through low-temperature aging and high-temperature roasting, wherein the low-temperature aging stage is kept warm for 1-3 hours while absorbing the gas, and then the temperature is increased. The high-temperature roasting stage is kept warm for 1-3 hours while volatilizing, cooling, and collecting the gas;

[0010] 4) crushing the roasted material obtained in step 3), and then leaching it with water, and separating the liquid and solid to obtain a water leaching liquid and corundum sand water leaching residue;

[0011] 5) subjecting the aqueous solution obtained in step 4) to hydrothermal carbonization, and liquid-solid separation to obtain a lithium carbonate product and a separated solution;

[0012] 6) returning the separated solution from step 5) to the water extract described in step 4) until the water extract solution reaches saturation, and obtaining sodium sulfate crystals through liquid-solid separation.

[0013] Preferably, the particle size of the overhaul slag after pretreatment in step 1) is less than 2 mm.

[0014] Preferably, in step 2), the pretreated overhaul slag and concentrated sulfuric acid are uniformly mixed in a mass ratio of 1.0:(1.0-1.5).

[0015] Preferably, in step 2), the mixing time of the pretreated overhaul slag and concentrated sulfuric acid is controlled to be 5-10 minutes.

[0016] Preferably, the temperature of the low-temperature aging in step 3) is 160-260°C, and the temperature of the high-temperature calcination is 800-1000°C.

[0017] Preferably, in step 3), NaAlO2 and NaOH are used as absorption liquids to absorb the gas to obtain dry Na3AlF6. The molar concentration ratio of NaAlO2 to NaOH in the absorption solution is 1.0 to 1.5:2.

[0018] Preferably, in step 3), the gas is volatilized, cooled, and collected to obtain concentrated sulfuric acid, which is returned to step 2) for use as raw material.

[0019] Preferably, the material in step 4) is crushed to less than 2 mm, leached with water, and then liquid-solid separation is performed according to a liquid-solid concentration mass ratio (6-12): 1 g / mL, a reaction temperature of 40-80°C, and a reaction time of 1-3 h to obtain a filtrate and a corundum sand product.

[0020] Preferably, in step 5), the hydrothermal carbonization treatment is to heat the water extract to a temperature in the range of 60-80° C., introduce carbon dioxide gas into the solution at a flow rate of 3-5 L / min, and control the reaction time to be 0.5-1.5 h.

[0021] The principle of this invention is that hazardous waste overhaul slag is used as raw material. Under the action of concentrated sulfuric acid, fluoride is volatilized as hydrogen fluoride at low temperatures, producing high-value cryolite that is returned to the aluminum electrolysis process. At high temperatures, the sulfuric acid volatilizes in a gaseous form, which is collected and returned to the batching process. Simultaneously, under high temperatures, aluminum compounds are oxidized in an oxygen atmosphere to form corundum sand, which is then separated by water leaching to obtain the corundum sand product. Lithium sulfate in the water leaching solution is then precipitated as lithium carbonate by passing it through carbon dioxide gas. The remaining solution undergoes a cyclic leaching process, resulting in the precipitation of saturated sodium sulfate crystals. The entire process is pollution-free, and the resulting products are all high-value products, with high economic benefits.

[0022] The main reaction formula of the mixed material in the tube furnace is:

[0023] 2Na3AlF6+6H2SO4=3Na2SO4+Al2(SO4)3+12HF↑

[0024] H2SO4=SO3↑+H2O↑

[0025] 2LiF+H2SO4=Li2SO4+2HF↑

[0026] The present invention has the following beneficial effects: By performing a two-stage roasting of overhaul slag, it effectively converts hazardous waste into high-value products (corundum sand, lithium carbonate, and cryolite). The entire process is effectively recycled, with no pollutants discharged and no residue generated. This not only eliminates the environmental hazards of overhaul slag storage but also fully utilizes the valuable elements it contains to produce high-value products, resulting in considerable economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0028] The following examples are intended to further illustrate the present invention, but are not intended to limit the scope of the present invention. Unless otherwise specified, the percentages are by weight.

[0029] Table 1 Main element composition and content of overhaul slag

[0030]

[0031] The method of the present invention can be used to recover and utilize the high-value elements Li, Al, and F in the form of Li2CO3, a-Al2O3, and Na3AlF6 products. Figure 1 .

[0032] from Figure 1 It can be seen that the pollution-free treatment and recycling method of electrolytic aluminum overhaul slag of the present invention includes the following steps:

[0033] 1. Pretreatment of overhaul slag: The overhaul slag is subjected to a crushing-screening process, screened with a 2mm sieve, and the material on the sieve is returned to the crushing process, and finally the material with a particle size of less than 2mm is obtained.

[0034] Using overhaul slag with particle size less than 2mm as raw material,

[0035] Take the overhaul slag raw material, add concentrated sulfuric acid to it and stir it evenly;

[0036] 2. Roasting: including aging stage and high temperature roasting stage

[0037] Curing stage: air is introduced at an air flow rate of 1-4 L / min, the temperature is raised to 160-260°C, and kept warm for 1-3 hours

[0038] The first stage gas output uses NaAlO2 and NaOH as the absorption liquid, and the mol concentration ratio of NaAlO2 to NaOH is 1.0-1.5:2. The first stage gas can be absorbed by the absorption liquid to obtain dry Na3AlF6;

[0039] High temperature roasting stage: the temperature is raised to 800-1000℃ and kept warm for 1-3 hours. After the second stage of gas production, the sulfuric acid evaporates in gaseous form and is directly cooled and collected for later use.

[0040] 3. Leaching: The roasted material is crushed to less than 2 mm and leached with water. Liquid-solid separation is performed according to the liquid-solid concentration mass ratio (6-12): 1 g / mL, the reaction temperature is 40-80 ° C, and the reaction time is 1-3 hours to obtain filtrate and corundum sand products.

[0041] The solution obtained by the leaching process is heated to a temperature in the range of 60-80°C, carbon dioxide gas is introduced into the solution at a flow rate of 3-5L / min, the reaction time is controlled at 0.5-1.5h, and then liquid-solid separation is performed to obtain a dry lithium carbonate product.

[0042] This treatment of overhaul slag yields both valuable materials and recyclable materials, ensuring a complete process with zero pollutant emissions and zero residue generation. Hazardous waste is transformed into high-value products such as corundum sand, lithium carbonate, and cryolite.

[0043] Implementation Case 1:

[0044] A process for high-value recycling of overhaul slag, according to Figure 1 The process shown includes the following steps:

[0045] Preparation of the mixture: Using the overhaul slag produced by an electrolytic aluminum plant in Hunan as raw material, take 50g of the pretreated overhaul slag raw material, add 50g of concentrated sulfuric acid into it, and stir it for 5 minutes to make it evenly stirred.

[0046] Calcination process: Place the uniformly mixed materials in a tubular furnace, introduce air at a flow rate of 2L / min, increase the temperature to 160℃ at a heating rate of 3℃ / min, and keep warm for 3h. The first stage of gas production uses NaAlO2 and NaOH in a ratio of 1.0:2.0 (mol) as the absorption liquid. The reaction temperature is 25℃. After the first stage of absorption is completed, 32.21g of dry Na3AlF6 can be obtained; then the temperature is further increased to 900℃ at a heating rate of 5℃ / min, and kept warm for 3h. After the second stage of gas production, it is directly cooled and collected for later use.

[0047] Leaching process: The roasted material was crushed to less than 2 mm and leached with water. The crushed material was leached at a liquid-solid ratio of 6:1 mL / g, a leaching temperature of 40°C, and a leaching time of 3 h. The solution was then liquid-solid separated to obtain 16.2 g of dry corundum sand.

[0048] Carbonization process: The solution obtained by the leaching process was heated to 60°C, and carbon dioxide was introduced at a flow rate of 3 L / min. After a reaction time of 1.5 h, liquid-solid separation was performed to obtain 3.62 g of dry lithium carbonate product.

[0049] Implementation Case 2:

[0050] A process for high-value recycling of overhaul slag, according to Figure 1 The process shown includes the following steps:

[0051] Preparation of the mixture: Using the overhaul slag produced by an electrolytic aluminum plant in Hunan as the raw material, take 50g of the pretreated overhaul slag raw material, add 60g of concentrated sulfuric acid into it, and stir it for 7 minutes to make it evenly stirred.

[0052] Calcination process: Place the uniformly mixed materials in a tubular furnace, introduce air at a flow rate of 3L / min, increase the temperature to 220℃ at a heating rate of 4℃ / min, and keep warm for 2h. The first stage of gas production uses NaAlO2 and NaOH in a ratio of 1.0:1.8 (mol) as the absorption liquid. The reaction temperature is 40℃. After the first stage of absorption is completed, 32.59g of dry Na3AlF6 can be obtained; then the temperature is further increased to 800℃ at a heating rate of 3℃ / min, and kept warm for 2h. After the second stage of gas production, it is directly cooled and collected for later use.

[0053] Leaching process: The roasted material was crushed to less than 2 mm, the crushed material was leached at a liquid-solid ratio of 8:1 mL / g, the leaching temperature was 60 ° C, the leaching time was 2 h, and then the solution was liquid-solid separated to obtain 17.0 g of dry corundum sand.

[0054] Carbonization process: The solution obtained by the leaching process was heated to 70°C, and carbon dioxide was introduced at a flow rate of 4 L / min. After a reaction time of 1.0 h, liquid-solid separation was performed to obtain 3.53 g of dry lithium carbonate product.

[0055] Implementation Case 3:

[0056] A process for high-value recycling of overhaul slag, according to Figure 1 The process shown includes the following steps:

[0057] Preparation of the mixture: Using the overhaul slag produced by an electrolytic aluminum plant in Hunan as the raw material, take 50g of the pretreated overhaul slag raw material, add 75g of concentrated sulfuric acid into it, and stir it for 10 minutes to make it stir evenly.

[0058] Calcination process: Place the uniformly mixed materials in a tubular furnace, introduce air at a flow rate of 4L / min, increase the temperature to 260℃ at a heating rate of 5℃ / min, and keep warm for 1h. The first stage of gas production uses NaAlO2 and NaOH in a ratio of 1.0:1.5 (mol) as the absorption liquid. The reaction temperature is 60℃. After the first stage of absorption is completed, 31.31g of dry Na3AlF6 can be obtained; then the temperature is further increased to 1000℃ at a heating rate of 6℃ / min, and kept warm for 1h. After the second stage of gas production, it is directly cooled and collected for later use.

[0059] Leaching process: The roasted material was crushed to less than 2 mm, the crushed material was leached at a liquid-solid ratio of 10:1 mL / g, the leaching temperature was 60 ° C, the leaching time was 2.5 h, and then the solution was liquid-solid separated to obtain 17.31 g of dry corundum sand.

[0060] Carbonization process: The solution obtained by the leaching process was heated to 80°C, and carbon dioxide was introduced at a flow rate of 5 L / min. After a reaction time of 1.0 h, liquid-solid separation was performed to obtain 3.79 g of dry lithium carbonate product.

[0061] As can be seen from the above examples, the method of the present invention essentially involves mixing overhaul slag with concentrated sulfuric acid, then producing cryolite and recycling the sulfuric acid through a stepwise volatilization mechanism of hydrogen fluoride and sulfur trioxide at high temperature. Under high-temperature conditions, aluminum compounds are converted into stable corundum sand, which is then separated through water leaching to obtain a lithium-containing aqueous solution. This solution is then conditioned with carbon dioxide to produce a lithium carbonate product, and the remaining liquid is recycled as a leachate until saturated sodium sulfate crystals precipitate to obtain a sodium sulfate product. This method effectively achieves resource utilization of all components of overhaul slag, with a simple, pollution-free production process and high economic benefits.

Claims

1. A method for pollution-free treatment and recycling of electrolytic aluminum overhaul slag, comprising the following steps: 1) The overhaul slag is crushed and screened to pre-treat it to obtain pre-treated overhaul slag; 2) uniformly mixing the pretreated overhaul slag obtained in step 1) with a set amount of concentrated sulfuric acid to obtain a mixed material; 3) The mixed material obtained in step 2) is placed in a tubular furnace for roasting, and a roasted material is obtained by low-temperature aging and high-temperature roasting. The low-temperature aging stage is kept warm for 1-3 hours while absorbing the gas, and then the temperature is increased. The high-temperature roasting stage is kept warm for 1-3 hours while the gas is volatilized, cooled, and collected. 4) crushing the roasted material obtained in step 3), and then leaching it with water, and separating the liquid and solid to obtain a water leaching liquid and corundum sand water leaching residue; 5) subjecting the aqueous solution obtained in step 4) to hydrothermal carbonization, and performing liquid-solid separation to obtain a lithium carbonate product and a separation solution; 6) returning the separated solution from step 5) to the water extract from step 4) until the water extract reaches saturation, and obtaining sodium sulfate crystals through liquid-solid separation; in, In step 3), the low-temperature aging temperature is 160-260° C., and the high-temperature calcination temperature is 800-1000° C.; in step 3), the gas in the low-temperature aging stage is absorbed using NaAlO2 and NaOH as an absorption solution to obtain dry Na3AlF6; the molar concentration ratio of NaAlO2 to NaOH in the absorption solution is 1.0-1.5:2; In step 4), the material is crushed to less than 2 mm, and water leaching is performed according to a liquid-solid ratio (6-12): 1 mL / g, a reaction temperature of 40-80°C, and a reaction time of 1-3 h, followed by liquid-solid separation to obtain a filtrate and corundum sand product; Step 5) The hydrothermal carbonization treatment is to heat the water extract to a temperature in the range of 60-80°C, introduce carbon dioxide gas into the solution at a flow rate of 3-5 L / min, and control the reaction time to be 0.5-1.5 h.

2. The pollution-free treatment and recycling method for electrolytic aluminum overhaul slag according to claim 1 is characterized in that: The particle size of the overhaul slag after the pretreatment in step 1) is less than 2 mm.

3. The pollution-free treatment and recycling method for electrolytic aluminum overhaul slag according to claim 1 is characterized in that: In the step 2), the pretreated overhaul slag and concentrated sulfuric acid are uniformly mixed in a mass ratio of 1.0: (1.0-1.5).

4. The method for pollution-free treatment and recycling of electrolytic aluminum overhaul slag according to claim 3, characterized in that In the step 2), the mixing time of the pretreated overhaul slag and concentrated sulfuric acid is controlled to be 5-10 minutes.

5. The pollution-free treatment and recycling method for electrolytic aluminum overhaul slag according to claim 1 is characterized in that: In step 3), the gas from the high-temperature roasting stage is volatilized, cooled, and collected to obtain concentrated sulfuric acid, which is returned to step 2) for use as raw material.

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