A method for recovering rare earth elements from fly ash by synergistic leaching of bacteria and CO2 mineralization

Abstract

The application discloses a method for synergistically leaching and CO2 mineralization of rare earth in fly ash by double bacteria, which comprises the following steps: mixing high-sulfur tailings and fly ash to form a heap, pre-leaching by using an organic acid solution, and then adopting mixed bacteria solution of Aspergillus niger and acidophilic iron-oxidizing bacteria to perform bioheap leaching; collecting two rare earth leaching solutions after the heap leaching and conveying the solutions to a separator to obtain a preliminarily purified leaching solution, then conveying the solution to a No. I fixed bed filter and adjusting the pH to 3, filtering, conveying the obtained No. I filtrate to a No. II fixed bed filter and adjusting the pH to 4, filtering; repeating the above operation until the pH is adjusted to 7, and finally conveying CO2 into a No. V fixed bed filter to precipitate rare earth ions, and filtering to obtain a rare earth carbonate precipitate. The application can improve the leaching rate of rare earth, the fixing effect of CO2 and the precipitation rate of rare earth mineralized by CO2, and additionally, can reduce the cost, achieve the purpose of energy saving, waste utilization and carbon negative, and achieve the purpose of coordination and unity.

Description

Technical Field

[0001] This invention belongs to the field of fly ash biometallurgical technology, specifically relating to a method for recovering rare earth elements from fly ash through synergistic leaching by dual bacteria and CO2 mineralization. Background Technology

[0002] Currently, the main uses of fly ash include manufacturing cement concrete, synthesizing zeolite, soil improvement, and wastewater treatment. However, the recovery and utilization of rare earth elements is insufficient, resulting in resource waste. High-sulfur tailings are solid wastes from the sorting of high-sulfur coal mines, and the long-term accumulation of high-sulfur tailings occupies land resources.

[0003] Rare earth elements in fly ash are mostly found in a stable aluminosilicate network structure, exhibiting strong corrosion resistance. Therefore, traditional single acid leaching methods (such as hydrochloric acid or sulfuric acid) present numerous problems during extraction. For example, single acid leaching typically requires high-temperature and high-pressure treatment, resulting in high acid consumption (10-30 tons / ton of fly ash), accounting for over 60% of the cost, and generating large amounts of wastewater containing heavy metals. Taking sulfuric acid leaching as an example, although sulfuric acid can destroy the mineral structure, it also dissolves large amounts of impurities such as aluminum and iron, increasing the difficulty of subsequent separation. Common methods for extracting rare earth elements from fly ash include direct acid leaching and roasting acid leaching. Chemical leaching methods usually require large amounts of chemical reagents to extract rare earth elements, which not only increases production costs but may also generate toxic waste and pollutants, negatively impacting the environment. Furthermore, chemical methods use high-concentration chemical reagents, requiring equipment materials with high corrosion resistance, increasing equipment maintenance costs.

[0004] Compared to traditional chemical leaching methods, bioleaching technology extracts metal elements through microbial action under mild conditions, reducing environmental impact and energy consumption. The bioleaching process is relatively simple, requiring no complex equipment or specialized technology, and is easy to operate. In summary, bioleaching offers numerous advantages, including environmental friendliness, low cost, wide applicability, high efficiency, biodiversity, and sustainability, making it a promising metal extraction method with broad development prospects and application potential. *Thiobacillus ferrooxidans* and *Thiobacillus thiooxidans* have been successfully used to extract rare earth elements from fly ash, achieving a leaching rate of approximately 60%. However, currently, bioleaching is primarily performed using single bacterial species, and the leaching rate still needs improvement.

[0005] The usual method for obtaining rare earth products from leachates is selective precipitation. This is based on the different solubilities of rare earth ions with other impurities and precipitants such as sodium hydroxide, oxalic acid, and sodium sulfate under different temperature and acidity conditions. However, to obtain higher-grade rare earth products, a higher pH value (pH≥8) is required. On the one hand, this requires a significant increase in the amount of precipitant to be added, resulting in a substantial increase in cost; on the other hand, the strongly alkaline solution also puts greater pressure on subsequent wastewater treatment and the environment at the discharge site.

[0006] With the development of carbon dioxide capture, utilization, and storage (CCUS) technology in recent years, it has been achieved to react CO2 with calcium and magnesium components in solid waste through carbonation to generate CaCO3 or MgCO3. This has successfully and stably solidified large quantities of CO2 in building materials for a long period, realizing the coordinated development of CO2 utilization and the environmental protection industry. This also provides strong support for using CO2 instead of carbonate precipitants to purify and separate rare earth elements in leachate. Summary of the Invention

[0007] The purpose of this invention is to provide a method for recovering rare earth elements from fly ash by synergistic leaching of two bacteria and CO2 mineralization. This method can improve the leaching rate of rare earth elements, the fixation effect of CO2, and the precipitation rate of rare earth elements mineralized by CO2. In addition, it can reduce costs and achieve the coordinated goal of energy saving, waste utilization, and carbon negativeing.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: a method for recovering rare earth elements from fly ash by synergistic leaching of bacteria and CO2 mineralization, comprising the following steps: S1, synergistic leaching of organic acids and bio-organic substances; S1-1. Mix high-sulfur tailings and fly ash to form a ore pile. Sprinkle organic acid solution evenly on the ore pile for pre-leaching, while providing an acidic environment for the subsequent bacterial solution. After the pre-leaching reaction is completed, rare earth leachate and leaching residue are obtained. S1-2. The bacterial solution is evenly sprinkled onto the leaching residue obtained in step S1-1 for bioleaching; the bacterial solution is a mixed solution of *Aspergillus niger* and *Thiobacillus acidophilus*; the inoculation ratio between *Aspergillus niger* and *Thiobacillus acidophilus* is (0.25-4):1; the number of microorganisms in the ore pile reaches at least 4 × 10⁻⁶. 14 Units / ton; S1-3. Air is delivered to the bottom of the leaching residue to provide oxygen and CO2 for the microorganisms; S1-4. After heap leaching for seven to eight months, collect the rare earth leachate and transport the rare earth leachate obtained in step S1-1 to the precipitation process. S2. Precipitation Process: The leachate is separated to remove impurities, resulting in a pre-purified leachate. This leachate is then fed to a fixed-bed filter (No. I) and the pH is adjusted to 3 using sodium hydroxide solution. After settling and settling, the leachate is filtered. The resulting filtrate (No. I) is fed to a fixed-bed filter (No. II) and the pH is adjusted to 4 using sodium hydroxide solution. After settling and settling, the leachate is filtered. The resulting filtrate (No. II) is fed to a fixed-bed filter (No. III) and the pH is adjusted to 5 using sodium carbonate solution. After settling and settling, the leachate is filtered. The resulting filtrate (No. III) is fed to a fixed-bed filter (No. IV) and the pH is adjusted to 6 using sodium carbonate solution. After settling and settling, the leachate (No. IV) is fed to a fixed-bed filter (No. V) and the pH is adjusted to 7 using sodium carbonate solution. Finally, CO2 gas is introduced into the fixed-bed filter (No. V) to precipitate rare earth ions. After precipitating for 4-6 hours, the precipitate is filtered to obtain a rare earth carbonate precipitate as the main product.

[0009] Preferably, in step S1-1, the concentration of the organic acid solution is 0.5-3 mol / L, the pH value of the organic acid solution is 1-3, the liquid-to-solid ratio of the organic acid solution to the ore pile is 5-20 mL / g, and the inorganic acid is one or more of maleic acid, malonic acid, tartaric acid, and lactic acid.

[0010] Preferably, in steps S1-2, the inoculation ratio between Aspergillus niger and Acidithiobacillus ferrooxidans is 7:3.

[0011] Preferably, in step S1-1, the mass ratio of fly ash to inorganic sulfur in the high-sulfur tailings in the ore pile is 83:1, and the total rare earth content in the fly ash is above 600 ppm.

[0012] Preferably, in steps S1-2, the number of viable bacteria in the bacterial solution is 1×10⁻⁶. 7 Up to 1×10 8 per ml.

[0013] Preferably, in step S2, the concentrations of sodium hydroxide solution and sodium carbonate solution are 0.05-0.08 mol / L and 0.1-0.3 mol / L, respectively; CO2 gas is introduced for 10-40 min.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention fixes CO2 twice, during the leaching and precipitation stages. During bioleaching, a certain amount of CO2 is consumed by the microbial strain's own carbon fixation physiological function, and CO2 will continue to be consumed in subsequent leaching stages for the growth and proliferation of the microbial strain. This invention uses CO2 instead of carbonate precipitant carbonic acid to purify and separate rare earth elements in the leachate. In the precipitation stage, a large amount of CO2 is consumed to complete the extraction of rare earth ions into rare earth carbonates. The process of this invention can consume a relatively large amount of CO2, achieving the goal of CO2 sequestration in the environment on a small scale, making a certain contribution to carbon neutrality, and is conducive to sustainable development.

[0015] (2) The present invention adopts a stepwise precipitation process. Sodium hydroxide and sodium carbonate have significant differences in precipitation and separation effects of rare earth elements and aluminum and iron in different pH ranges. Therefore, under the condition of room temperature of 25°C, sodium hydroxide and sodium carbonate are added in different pH ranges. After adjusting the pH of the solution to a specific value with a precipitant, the solution is allowed to stand for a period of time to precipitate. After the reaction is complete, the solution is filtered. When the pH is adjusted to 7, the rare earth precipitation rate is as high as 96%, and the rare earth grade is as high as 4.3%, which makes it easy to separate the rare earth.

[0016] (3) The present invention uses bidirectional combined leaching of acidophilic sulfur-oxidizing bacteria and Aspergillus niger, which greatly improves the leaching rate of rare earth ions.

[0017] (4) This invention makes full use of two types of solid waste: fly ash and high-sulfur tailings. Fly ash is used as an industrial raw material, and high-sulfur tailings are used as nutrients for microbial inoculum to extract rare earth elements. This invention solves the problem that fly ash cannot provide energy for microorganisms, eliminating the need to add elemental sulfur, pyrite, or other substances, thus minimizing costs and solving the environmental pollution problem of high-sulfur tailings. In addition, this invention provides a certain channel for the resource utilization of fly ash.

[0018] (5) The present invention has low energy consumption, low chemical reagent consumption and low cost; the main economic consumable in the present invention is NaOH, which greatly reduces the cost compared with chemical leaching, and the content of rare earth elements extracted is not significantly different, which has a high cost performance and feasibility.

[0019] In summary, this invention features simple process control, improves the leaching rate of rare earth elements, enhances CO2 fixation, and increases the precipitation rate of rare earth mineralization using CO2. Furthermore, it reduces production costs, achieving a harmonious balance between energy conservation, waste utilization, and carbon neutrality. Moreover, this invention fully utilizes three types of coal mine waste—high-sulfur tailings, fly ash, and other coal-mining waste materials—to recover rare earth elements from fly ash, minimizing costs and addressing the environmental problems caused by these materials. This achieves the goal of turning waste into treasure and treating waste with waste. Attached Figure Description

[0020] Figure 1This is a schematic diagram of the process flow of the present invention; Figure 2 This is a schematic diagram showing the effect of rare earth ion leaching rate on the inoculation ratio between Comparative Examples 3-4 and Examples 1-5. Figure 3 This is a statistical diagram showing the recovery rate and grade of impurity ions and rare earth REEs under different pH conditions and different precipitant addition points in Example 1; wherein: sodium hydroxide solution is added when pH < 5, and sodium carbonate solution is added when pH 5~10; Figure 4 This is a statistical diagram illustrating the recovery rate and grade of impurity ions and rare earth REEs under different pH conditions and different precipitant addition points in Comparative Example 5; where sodium hydroxide solution was added when pH < 3, and sodium carbonate solution was added when pH 3 ~ 7. Figure 5 This is a statistical diagram illustrating the recovery rate and grade of impurity ions and rare earth REEs under different pH conditions using sodium hydroxide solution as a precipitant in Comparative Example 6. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Example 1 like Figure 1 As shown, a method for recovering rare earth elements from fly ash through synergistic leaching by bacteria and CO2 mineralization is presented. The fly ash used in this embodiment is from a power plant in Datong, Shanxi Province, and contains 680 ppm of rare earth elements and 3.2% of calcium. The sulfur content of a high-sulfur tailings from a certain flotation coal mine is 12%. The method includes the following steps: S1, synergistic leaching of organic acids and bio-organic substances; S1-1. Mix high-sulfur tailings and fly ash to form a ore pile. Sprinkle a 2 mol / L maleic acid solution with a pH of 1-3 evenly on the ore pile for pre-leaching. The liquid-to-solid ratio of the maleic acid solution to the ore pile is 15 mL / g. At the same time, it provides an acidic environment for the subsequent bacterial solution. After the pre-leaching reaction is completed, rare earth leachate and leachate residue are obtained. S1-2. The bacterial solution is evenly sprinkled onto the leaching residue obtained in step S1-1 for bioleaching; the bacterial solution is a mixed bacterial solution of Aspergillus niger and Acidithiobacillus ferrooxidans, with a viable count of 1×10⁻⁶. 8 The inoculation ratio between *Aspergillus niger* and *Thiobacillus ferrooxidans* was 7:3; the number of microorganisms in the ore pile reached at least 4 × 10⁻⁶. 14 Units / ton; Acidophilic ferrooxidizing thiobacillus produces sulfuric acid by oxidizing sulfur in tailings and decomposes fly ash to release rare earth ions; S1-3. Air is delivered to the bottom of the leaching residue by a compressor to provide oxygen and CO2 for the microorganisms; S1-4. After seven to eight months of heap leaching, place the collection pipe at the bottom of the ore heap to collect the leachate, allowing it to flow into the collection pool for storage. Collect the rare earth leachate and transport the rare earth leachate obtained in step S1-1 to the precipitation process. S2. Precipitation Process: The leachate is separated to remove impurities, resulting in a pre-purified leachate. This leachate is then fed to a fixed-bed filter (No. I) and the pH is adjusted to 3 using sodium hydroxide solution. After settling and filtration, the resulting filtrate (No. I) is fed to a fixed-bed filter (No. II) and the pH is adjusted to 4 using sodium hydroxide solution. After settling and filtration, the resulting filtrate (No. II) is fed to a fixed-bed filter (No. III) and the pH is adjusted to 5 using sodium carbonate solution. After settling and filtration, the resulting filtrate (No. III) is fed to a fixed-bed filter (No. IV) and the pH is adjusted to 6 using sodium carbonate solution. After settling and filtration, the resulting filtrate (No. IV) is fed to a fixed-bed filter (No. V) and the pH is adjusted to 7 using sodium carbonate solution. Finally, CO2 gas is introduced into the fixed-bed filter (No. V) to precipitate rare earth ions. After 4-6 hours of precipitation, filtration yields a precipitate of rare earth carbonate as the main product. Impurity ion precipitates (Fe(OH)3, Al(OH)3, and some Ca(OH)2) can be obtained from fixed-bed filters (No. I, II, III, and IV).

[0023] like Figure 2 and Figure 3 As shown, this embodiment processes 100,000 tons of fly ash annually, consuming 10,000 tons of high-sulfur tailings. The rare earth leaching rate reaches 82%, the rare earth REE precipitation rate reaches 96%, and at pH 7, the precipitation rate of byproducts aluminum reaches 96%, the precipitation rate of iron reaches 82%, and the rare earth REE grade reaches a maximum of 4.3%. Annual recovery of rare earth oxides is 48.4 tons (equivalent to 83.7 tons of rare earth carbonate and 53.6 tons of fixed CO2), with simultaneous recovery of 18.3 tons of calcium carbonate (fixing 8.1 tons of CO2). The bioleaching stage uses 100 million liters of bacterial solution (1×10⁻⁶). 8 The process (using a fraction of a certain number of particles per mL) consumes 101.4 tons of CO2 within 12 days, with a total CO2 fixation of 163.1 tons over the entire process. It produces 2800 tons of iron-aluminum hydroxide and 12,000 tons of calcium sulfate as byproducts, generating economic benefits of 40.75 million yuan (including 38.72 million yuan in rare earth revenue and 16,300 yuan in carbon sequestration revenue). Simultaneously, it reduces fly ash storage by 100,000 tons and SO2 emissions by 1192 tons, achieving synergistic effects of solid waste resource utilization and carbon emission reduction.

[0024] Example 2 like Figure 1 As shown, a method for recovering rare earth elements from fly ash by synergistic leaching of bacteria and CO2 mineralization is described. The fly ash and high-sulfur tailings used in this embodiment are the same as those in Example 1. The only difference between the method steps is that steps S1-1 and S1-2. The other steps are the same as those in Example 1.

[0025] In this embodiment, the organic acid solution in step S1-1 is tartaric acid solution, and the inoculation ratio between Aspergillus niger and Acidithiobacillus acidophilus in step S1-2 is 4:1.

[0026] like Figure 2 As shown, this comparative example processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, and achieves a rare earth leaching rate of 78%.

[0027] Example 3 like Figure 1 As shown, a method for recovering rare earth elements from fly ash by synergistic leaching of bacteria and CO2 mineralization is described. The fly ash and high-sulfur tailings used in this embodiment are the same as those in Example 1. The only difference between the method steps is that steps S1-1 and S1-2. The other steps are the same as those in Example 1.

[0028] In this embodiment, the organic acid solution in step S1-1 is malonic acid solution, and the inoculation ratio between Aspergillus niger and Acidithiobacillus acidophilus in step S1-2 is 3:7.

[0029] like Figure 2 As shown, this comparative example processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, and achieves a rare earth leaching rate of 78%.

[0030] Example 4 like Figure 1 As shown, a method for recovering rare earth elements from fly ash by synergistic leaching of bacteria and CO2 mineralization is described. The fly ash and high-sulfur tailings used in this embodiment are the same as those in Example 1. The only difference between the method steps is that steps S1-1 and S1-2. The other steps are the same as those in Example 1.

[0031] In this embodiment, the organic acid solution in step S1-1 is a lactic acid solution, and the inoculation ratio between Aspergillus niger and Acidithiobacillus acidophilus in step S1-2 is 1:4.

[0032] like Figure 2 As shown, this comparative example processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, and achieves a rare earth leaching rate of 75%.

[0033] Comparative Example 1 A method for recovering rare earth elements from fly ash. The fly ash and high-sulfur tailings used in this comparative example are the same as those in Example 1. Steps S1-2 are omitted from the method steps, while the other steps remain the same as those in Example 1.

[0034] This facility processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, and achieves a rare earth leaching rate of 65%.

[0035] Comparative Example 2 A method for recovering rare earth elements from fly ash. The fly ash and high-sulfur tailings used in this comparative example are the same as those in Example 1. The method does not include step S1-1, and the other steps are the same as those in Example 1.

[0036] This facility processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, and achieves a rare earth leaching rate of 70%.

[0037] Comparative Example 3 A method for recovering rare earth elements from fly ash. The fly ash and high-sulfur tailings used in this comparative example are the same as those in Example 1. The only difference in the method steps is step S1-2, while the other steps remain the same as those in Example 1.

[0038] The specific process of step S1-2 in this comparative example is as follows: The Aspergillus niger bacterial solution (with a viable count of 1×10⁻⁶) is prepared... 8 The sample (each 1 ml) was evenly sprinkled onto the leaching residue obtained in step S1-1 for bio-heap leaching.

[0039] like Figure 2 As shown, this comparative example processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, and achieves a rare earth leaching rate of 67%.

[0040] Comparative Example 4 A method for recovering rare earth elements from fly ash. The fly ash and high-sulfur tailings used in this comparative example are the same as those in Example 1. The only difference in the method steps is step S1-2, while the other steps remain the same as those in Example 1.

[0041] The specific process of step S1-2 in this comparative example is as follows: high-sulfur tailings and fly ash are mixed to form a ore pile, and acidophilic ferrooxidizobacterium oxygenase (live bacteria count of 1×10⁻⁶) bacterial solution is added. 8 The sample (each 1 ml) was evenly sprinkled onto the leaching residue obtained in step S1-1 for bio-heap leaching.

[0042] like Figure 2 As shown, this comparative example processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, and achieves a rare earth leaching rate of 73%.

[0043] Comparative Example 5 A method for recovering rare earth elements from fly ash. The fly ash and high-sulfur tailings used in this comparative example are the same as those in Example 1. The only difference in the method steps is step S2. In step S2, sodium hydroxide is used to precipitate when pH < 3, and sodium carbonate solution is used to adjust the pH value when pH is 3-7. All other steps are the same as those in Example 1.

[0044] like Figure 2 and Figure 4As shown, this comparative example processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, achieves a rare earth leaching rate of 82%, a rare earth REE precipitation rate of 97%, and at a pH of 7, achieves a precipitation rate of 94% for aluminum and 93% for iron, with the rare earth REE grade reaching a maximum of 0.2%.

[0045] Comparative Example 6 A method for recovering rare earth elements from fly ash. The fly ash and high-sulfur tailings used in this comparative example are the same as those in Example 1. The only difference in the method steps is step S2. In step S2, sodium hydroxide solution is used to adjust the pH value. The other steps are the same as those in Example 1.

[0046] like Figure 2 and Figure 5 As shown, this comparative example processes 100,000 tons of fly ash annually, consumes 10,000 tons of high-sulfur tailings, achieves a rare earth leaching rate of 82%, a rare earth REE precipitation rate of 98%, and at a pH of 7, achieves a precipitation rate of 98% for aluminum and 92% for iron, with the rare earth REE grade reaching a maximum of 0.4%.

[0047] like Figure 3-5As shown, compared with Comparative Examples 5 and 6, in Example 1, considering the selectivity of the precipitation range, sodium hydroxide precipitation was used when pH < 5 in Example 1. Aluminum and iron ions, due to their high charge density and strong hydrolysis ability, preferentially formed hydroxide precipitates in this range. As can be observed from the figure, when pH ≤ 5, the recovery rates of Al and Fe significantly increased and tended to complete precipitation, while the recovery rate of rare earth elements (REEs) was extremely low in this range, indicating that REEs were almost not precipitated and remained largely in the solution. This created conditions for subsequent selective precipitation of REEs. When sodium carbonate precipitation was used at pH 5-10, REEs precipitated as carbonates. At this point, Al and Fe were essentially removed, avoiding co-precipitation interference and resulting in a higher grade of the REE precipitation product. In Comparative Example 5, sodium hydroxide was used for precipitation at pH < 3. Although Al and Fe began to precipitate, REE showed some precipitation at pH 4-5 (as shown in the figure, the REE recovery rate curve rises in this range), indicating that the precipitation ranges of REE and Al / Fe overlapped, leading to incomplete separation and a decrease in the REE grade of the product. In Comparative Example 6, sodium hydroxide was used for precipitation at pH 0-7. The precipitation range of Al and Fe was prolonged, and the overlap between the precipitation ranges of REE and Al / Fe increased (the figure shows that REE had a significant recovery rate at pH 5-7), further exacerbating the co-precipitation phenomenon and reducing the separation efficiency of REE and Al / Fe, as well as the product grade. From the perspective of product grade, in Example 1, since Al and Fe were basically completely precipitated at pH ≤ 5, there were fewer impurity ions in the solution during subsequent REE precipitation, resulting in a significant increase in the grade of the REE product (Grade curve in the figure) after pH 5, which remained at a high level. In Comparative Examples 5 and 6, due to the overlap of the precipitation ranges of REE and Al / Fe, some Al and Fe were mixed into the REE precipitate, resulting in a lower REE product grade than in Example 1. In summary, Example 1 achieved excellent selective separation of Al, Fe and REE precipitation processes at different pH ranges by using sodium hydroxide and sodium carbonate as precipitants, reducing co-precipitation interference and thus obtaining higher-grade rare earth products. The precipitation effect was better than that of Comparative Examples 5 and 6. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for recovering rare earth elements from fly ash by synergistic leaching of bacteria and CO2 mineralization, characterized in that, Includes the following steps: S1, synergistic leaching of organic acids and bio-organic substances; S1-1. Mix high-sulfur tailings and fly ash to form a ore pile. Sprinkle organic acid solution evenly on the ore pile for pre-leaching, while providing an acidic environment for the subsequent bacterial solution. After the pre-leaching reaction is completed, rare earth leachate and leaching residue are obtained. S1-2. The bacterial solution is evenly sprinkled onto the leaching residue obtained in step S1-1 for bioleaching; the bacterial solution is a mixed solution of *Aspergillus niger* and *Thiobacillus acidophilus*; the inoculation ratio between *Aspergillus niger* and *Thiobacillus acidophilus* is (0.25-4):1; the number of microorganisms in the ore pile reaches at least 4 × 10⁻⁶. 14 Units / ton; S1-3. Air is delivered to the bottom of the leaching residue to provide oxygen and CO2 for the microorganisms; S1-4. After heap leaching for seven to eight months, collect the rare earth leachate and transport the rare earth leachate obtained in step S1-1 to the precipitation process. S2. Precipitation Process: The leachate is separated to remove impurities, resulting in a pre-purified leachate. This leachate is then fed to a fixed-bed filter (No. I) and the pH is adjusted to 3 using sodium hydroxide solution. After settling and settling, the leachate is filtered. The resulting filtrate (No. I) is fed to a fixed-bed filter (No. II) and the pH is adjusted to 4 using sodium hydroxide solution. After settling and settling, the leachate is filtered. The resulting filtrate (No. II) is fed to a fixed-bed filter (No. III) and the pH is adjusted to 5 using sodium carbonate solution. After settling and settling, the leachate is filtered. The resulting filtrate (No. III) is fed to a fixed-bed filter (No. IV) and the pH is adjusted to 6 using sodium carbonate solution. After settling and settling, the leachate (No. IV) is fed to a fixed-bed filter (No. V) and the pH is adjusted to 7 using sodium carbonate solution. Finally, CO2 gas is introduced into the fixed-bed filter (No. V) to precipitate rare earth ions. After precipitating for 4-6 hours, the precipitate is filtered to obtain a rare earth carbonate precipitate as the main product.

2. The method for recovering rare earth elements from fly ash by synergistic leaching and CO2 mineralization using dual bacteria, as described in claim 1, is characterized in that... In step S1-1, the concentration of the organic acid solution is 0.5-3 mol / L, and the pH value of the organic acid solution is 1-3; the liquid-to-solid ratio of the organic acid solution to the ore pile is 5-20 mL / g, and the organic acid is one or more of maleic acid, malonic acid, tartaric acid, and lactic acid.

3. The method for recovering rare earth elements from fly ash by synergistic leaching and CO2 mineralization according to claim 1 or 2, characterized in that, In steps S1-2, the inoculation ratio between Aspergillus niger and Acidithiobacillus ferrooxidans is 7:

3.

4. A method for recovering rare earth elements from fly ash by synergistic leaching and CO2 mineralization according to claim 1 or 2, characterized in that, In step S1-1, the mass ratio of fly ash to inorganic sulfur in the high-sulfur tailings in the ore pile is 83:1, and the total rare earth content in the fly ash is above 600 ppm.

5. A method for recovering rare earth elements from fly ash by synergistic leaching and CO2 mineralization according to claim 1 or 2, characterized in that, In steps S1-2, the number of viable bacteria in the bacterial solution is 1×10⁻⁶. 7 Up to 1×10 8 per ml.

6. A method for recovering rare earth elements from fly ash by synergistic leaching and CO2 mineralization according to claim 1 or 2, characterized in that, In step S2, the concentrations of sodium hydroxide solution and sodium carbonate solution are 0.05-0.08 mol / L and 0.1-0.3 mol / L, respectively; CO2 gas is introduced for 10-40 min.

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