Method for extracting rare earth elements from ardealite

Abstract

The application discloses a method for extracting rare earth elements from phosphogypsum, and belongs to the technical field of phosphogypsum resource utilization and rare earth recovery. The method comprises the following steps: mixing rare earth-containing phosphogypsum with sodium carbonate and performing roasting to obtain heat-activated phosphogypsum; mixing the heat-activated phosphogypsum with biogas residue and biomass auxiliary materials, and adding a phosphorus-phenolic carboxylic acid enrichment functional composite microbial inoculum to perform aerobic fermentation, so as to obtain an ecological organic fertilizer containing low-molecular aromatic phenolic carboxylic acid; applying the ecological organic fertilizer to a planting substrate, and planting Dicranopteris dichotoma, Blechnum spicant or Eupatorium fortunei, so that the plants absorb and enrich rare earth elements; collecting the plants, and performing drying, ashing, hydrochloric acid leaching, ammonium oxalate precipitation and calcination to obtain rare earth oxides. The application realizes the synergistic resource utilization of phosphogypsum, organic solid waste and biomass waste, and reduces the material quantity and separation load of subsequent acid leaching treatment.

Description

Technical Field

[0001] This invention relates to the field of phosphogypsum resource utilization and rare earth recovery technology, specifically to a method for extracting rare earth elements from phosphogypsum. Background Technology

[0002] Phosphogypsum is a solid byproduct generated during the wet-process production of phosphoric acid and phosphate fertilizers. Influenced by the composition of phosphate rock and the production process, phosphogypsum may contain unreacted phosphate rock, phosphates, carbonates, silicates, and associated rare earth elements, in addition to calcium sulfate. Long-term stockpiling of phosphogypsum not only occupies land but also easily causes the migration of phosphorus-containing components and soluble impurities. Therefore, its reduction, harmlessness, and resource utilization are of great significance.

[0003] Existing methods for recovering rare earth elements from phosphogypsum mainly include acid leaching, alkali conversion leaching, roasting-leaching, and solvent extraction. Due to the low content of rare earth elements, and the fact that some rare earth elements exist in lattice-encapsulated, phosphate-bound, or associated mineral states, direct strong acid leaching of large quantities of phosphogypsum usually results in problems such as high reagent consumption, significant dissolution of matrix impurities, and high subsequent separation load of the leachate.

[0004] Although plant extraction can utilize rhizosphere acidification, complexation, and absorption to transfer metal elements dispersed in a solid matrix into the plant, the accessibility of rare earth elements in unactivated phosphogypsum is low, and the lack of organic matter and rhizosphere active substances required for plant growth limits the enrichment efficiency. Therefore, a new extraction process is needed to extract rare earth elements from phosphogypsum. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for extracting rare earth elements from phosphogypsum.

[0006] A method for extracting rare earth elements from phosphogypsum includes the following steps: S1. Rare earth phosphogypsum is mixed with sodium carbonate and calcined to cause thermal decomposition and mineral phase reconstruction of the associated minerals in the phosphogypsum, resulting in thermally activated phosphogypsum. S2. Mix biogas residue, the thermally activated phosphogypsum and biomass additives, adjust the moisture content of the mixture, add phosphorus-solubilizing phenolic carboxylic acid enrichment functional compound microbial agent at the initial stage of aerobic fermentation and carry out aerobic fermentation to obtain ecological organic fertilizer containing low molecular weight aromatic phenolic carboxylic acid. S3. Apply the ecological organic fertilizer to the planting substrate and plant plants, so that the plants can absorb and enrich rare earth elements from phosphogypsum. S4. Collect the plants obtained in step S3, dry and ashed to obtain plant ash; acid leaching of the plant ash to separate rare earth-containing leachate, and oxalate precipitation and calcination of the rare earth-containing leachate to obtain rare earth oxides.

[0007] In step S1, the amount of sodium carbonate added is 4-6% of the mass of rare earth phosphogypsum, the calcination temperature is 800-900℃, and the calcination time is 100-150min.

[0008] In step S2, the mass ratio of the biogas residue, thermally activated phosphogypsum and biomass additives is (11-15):(3-5):(6-7), and the moisture content of the mixture is 55-60 wt%.

[0009] The biomass feedstock is selected from one or more of corn stalks, rice husks, and garden waste.

[0010] The phosphorus-solubilizing-phenolic carboxylic acid enrichment functional compound microbial agent comprises, by dry weight of microorganisms, 10-20 parts of Aspergillus niger, 5-15 parts of Penicillium oxalate, 1-5 parts of white-rot fungi, 15-20 parts of Pseudomonas, 3-10 parts of Bacillus, 5-18 parts of Actinomycetes and 12-15 parts of Streptomyces.

[0011] The preparation method of the phosphorus-solubilizing-phenolic carboxylic acid enrichment functional compound bacterial agent includes: activating each bacterial strain separately in phosphorus-solubilizing medium, mixing them, and culturing at 25-30℃ for 48-72h.

[0012] In step S2, the amount of the functional compound microbial agent added is 1-5% of the volatile solids mass of the mixture; the air supply for aerobic fermentation is 0.1-0.3 L / min / kg, the temperature during the high-temperature period is greater than 55℃ and maintained for at least 5 days, and the total fermentation time is 35-49 days.

[0013] In step S3, the plant with rare earth enrichment ability is selected from one or more of the following: *Dictamnus dasycarpus*, *Adiantum capillus-veneris*, and *Centipeda minima*; the ecological organic fertilizer has a mass fraction of 18-25% in the planting substrate, and the planting time is 30 days.

[0014] In step S4, the ashing is carried out at 600℃ for 30 minutes; the acid leaching is carried out with 1 mol / L hydrochloric acid, and the solid-liquid separation is performed after acid leaching to obtain the rare earth-containing leachate.

[0015] The pH of the rare earth-containing leachate is adjusted to 4.5-5.5, and ammonium oxalate is added to form rare earth oxalate precipitate. The rare earth oxalate precipitate is washed, dried, and then calcined at 800-900℃ for 1-3 hours to obtain the rare earth oxide.

[0016] Due to the adoption of the above technical solutions, the beneficial effects of the present invention include: (1) In this invention, sodium carbonate is used to assist in high-temperature calcination to thermally activate rare earth phosphogypsum, causing associated carbonate minerals to undergo thermal decomposition and promoting the dehydration and mineral phase reconstruction of calcium sulfate and phosphate minerals. This increases the cracks, pores and active sites on the surface of phosphogypsum particles, exposing the rare earth phases that are fixed by the mineral lattice or encapsulation structure, and promoting the transformation of rare earth elements into forms that are easily affected by organic acids, easily migrate to the rhizosphere and are easily absorbed by plants.

[0017] (2) This invention involves the synergistic aerobic fermentation of thermally activated phosphogypsum with biogas residue and biomass adjuvants, and the addition of a phosphorus-solubilizing and phenolic carboxylic acid enrichment compound microbial agent. Aspergillus niger, Penicillium oxalate, Pseudomonas, and Bacillus can produce low-molecular-weight organic acids such as oxalic acid, citric acid, and malic acid, promoting the dissolution of insoluble phosphates and the migration and release of rare earth elements. White-rot fungi, actinomycetes, and Streptomyces participate in the degradation of lignocellulose and aromatic organic matter in biomass, which is beneficial to the formation and accumulation of low-molecular-weight aromatic phenolic carboxylic acids such as coumaric acid, salicylic acid, ferulic acid, and caffeic acid. Therefore, the resulting ecological organic fertilizer has the functions of organic matter supply, phosphorus activation, and rhizosphere regulation.

[0018] (3) This invention utilizes low-molecular-weight aromatic phenolic carboxylic acids formed or enriched in ecological organic fertilizers to regulate the rhizosphere environment of plants. These low-molecular-weight aromatic phenolic carboxylic acids can promote the development of lateral roots and root hairs, expand the contact area between plant roots and the planting substrate, and facilitate the colonization of rhizosphere microorganisms and the cycling of rhizosphere organic acids, thereby improving the plant's ability to absorb phosphorus and related mineral elements. During the process of plant absorption and activation of phosphorus, rare earth elements in the planting substrate, which have high mobility after thermal activation and microbial action, can simultaneously enter the plant body, thereby enabling *Dicranopteris dichotoma*, *Adiantum capillus-veneris*, or *Centipeda minima* to absorb and enrich rare earth elements from phosphogypsum.

[0019] (4) This invention does not directly extract large amounts of phosphogypsum with strong acid. Instead, it first transfers the low-content, dispersed rare earth elements in the phosphogypsum and planting substrate to the smaller volume of plant biomass through plant absorption. Then, the water and most of the organic matter in the plant are removed through drying and ashing, further concentrating the rare earth elements in the plant ash. This "plant transfer-ashing concentration" treatment method can significantly reduce the amount of solid material entering the subsequent acid leaching process, reduce the interference of matrix components such as calcium sulfate, phosphate, organic matter and soil minerals on acid leaching and precipitation separation, thereby reducing the load of subsequent wet processing and improving the purity of rare earth recovery products.

[0020] (5) In this invention, hydrochloric acid is used to leach plant ash, and ammonium oxalate is added at a pH of 4.5-5.5 to form rare earth oxalate precipitate. After washing, drying and calcination, stable rare earth oxides are obtained. This recovery route is clear to operate, has a low acid concentration, and the oxalate precipitation is beneficial for the separation of rare earth elements from some soluble impurities.

[0021] (6) This invention constructs a continuous process of "sodium carbonate-assisted thermal activation - aerobic fermentation with functional compound microbial agents - plant absorption and enrichment - ashing and acid leaching - oxalate precipitation and calcination", realizing the synergistic resource utilization of phosphogypsum, biogas residue, corn stalks, rice husks, or garden waste. The functions of each step are progressive and interconnected, which can improve the mobility, enrichment and recyclability of rare earth elements in low-grade phosphogypsum, and reduce the reagent consumption and separation load caused by direct strong acid leaching of large amounts of phosphogypsum.

[0022] This invention simultaneously improves the bioavailability of phosphorus and the extraction rate of rare earth elements in the preparation of eco-friendly phosphate fertilizer from phosphogypsum. The consumption of acid / alkali reagents is significantly reduced compared to the traditional acid hydrolysis method, and the purity of rare earth elements is increased by more than 40%. Compared to the rare earth element content in phosphorus and rare earth associated minerals, the amount of rare earth elements extracted by this method is increased by at least five times. Attached Figure Description

[0023] Figure 1 Figure showing the content of low molecular weight aromatic phenolic carboxylic acids in the ecological organic fertilizer obtained by roasting and fermenting phosphogypsum at different temperatures.

[0024] Figure 2 The XRD pattern of the uncalcined phosphogypsum sample is shown.

[0025] Figure 3 The image shows the XRD pattern of thermally activated phosphogypsum obtained by calcining phosphogypsum at 500℃.

[0026] Figure 4 The image shows the XRD pattern of thermally activated phosphogypsum obtained by calcining phosphogypsum at 850℃.

[0027] Figure 5 SEM images of phosphogypsum at different calcination temperatures. Detailed Implementation

[0028] The present invention will be further described below with reference to preparation examples and embodiments, so that those skilled in the art can understand and implement the present invention. However, the following preparation examples and embodiments are not intended to limit the scope of protection of the present invention.

[0029] Unless otherwise stated, the feed weights in this instruction manual are calculated based on the dry basis weight of the raw materials; the moisture content refers to the percentage of the water mass in the material to the total wet basis weight of the material; the room temperature is 23±2℃; for operations involving pH adjustment, the pH value shall be controlled within the range of ±0.2 of the target pH value.

[0030] I. Preparation of Phosphate-Solubilizing-Phenolic Carboxylic Acid Enrichment Functional Compound Microbial Agent 1. Preparation of phosphorus-solubilizing medium The phosphorus-solubilizing medium was purchased from Shanghai Titan Technology Co., Ltd., NBRIP medium powder, model number 0415939367. 25g of NBRIP medium powder was weighed and added to approximately 1.0L of deionized water. The mixture was stirred until all components were fully dissolved or evenly dispersed. The pH was adjusted to 7.0, and then brought to a final volume of 1.2L with deionized water. After dispensing, the mixture was sterilized at 121℃ for 20 minutes and cooled to room temperature before use.

[0031] All strains used were those listed in Table 1 of the instruction manual. For Aspergillus niger, Penicillium oxalate, and white-rot fungi, the inoculum quantity was calculated based on spore count, ensuring an initial spore concentration of 1 × 10⁻⁶ in the culture medium. 6 / mL; the inoculation amount of Pseudomonas, Bacillus, Actinomycetes and Streptomyces is based on viable count, so that the initial viable concentration in the culture medium is 1×10⁻⁶. 6 CFU / mL. The specific amount of freeze-dried bacterial powder to be weighed should be determined based on the live bacteria count or spore count indicated on the freeze-dried powder label.

[0032] Aspergillus niger, Penicillium oxalate, white-rot fungi, Pseudomonas, Bacillus, Actinomycetes, and Streptomyces were all derived from the China General Microbiological Culture Collection Center. Specific information is shown in Table 1.

[0033] Table 1

[0034] 2. Preparation of Phosphate-Solubilizing Compound Microbial Agent Preparation Example 1 This preparation example provides a phosphorus-solubilizing and phenolic carboxylic acid enrichment functional compound bacterial agent, wherein the dry weight ratio of each bacterial species is as follows: Aspergillus niger : Penicillium oxalate : White rot fungi : Pseudomonas : Bacillus : Actinomycetes : Streptomyces = 10 : 5 : 1 : 15 : 3 : 5 : 12.

[0035] (1) Activation of each strain Take 1.0L of sterile NBRIP liquid medium and place it in a 3L Erlenmeyer flask with baffle. Inoculate Aspergillus niger, Penicillium oxalate, white rot fungi, Pseudomonas, Bacillus, Actinomycetes and Streptomyces into the corresponding NBRIP liquid medium respectively.

[0036] Aspergillus niger, Penicillium oxalate, and white rot fungi were cultured with shaking at 28℃ and 150 rpm for 72 h; Pseudomonas and Bacillus were cultured with shaking at 28℃ and 160 rpm for 48 h; and Actinomycetes and Streptomyces were cultured with shaking at 28℃ and 150 rpm for 72 h to obtain activated cultures of each species.

[0037] (2) Collection of bacterial cells and adjustment of dry weight concentration Filter the activated cultures of Aspergillus niger, Penicillium oxalate, and white rot fungi separately and collect the mycelium; centrifuge the activated cultures of Pseudomonas and Bacillus at 6000×g for 10 min and collect the cells; centrifuge the activated cultures of Actinomycetes and Streptomyces at 5000×g for 10 min and collect the cells.

[0038] The obtained bacterial cells were washed twice with sterile 0.85wt% sodium chloride aqueous solution. Each bacterial cell suspension sample was then vacuum-dried at 60℃ to constant weight, and the dry weight concentration of the cells was determined. The dry weight concentration of the concentrated bacterial cell suspensions of each strain was uniformly adjusted to 20 g / L using sterile 0.85wt% sodium chloride aqueous solution.

[0039] (3) Preparation of compound microbial agents Take 980 mL of concentrated Aspergillus niger cell suspension, corresponding to a dry cell weight of 19.6 g; 490 mL of concentrated Penicillium oxalate suspension corresponds to a dry weight of 9.8 g of cells; 100 mL of concentrated white-rot fungal suspension corresponds to a dry weight of 2.0 g of fungal cells; 1470 mL of concentrated Pseudomonas cell suspension corresponds to a dry cell weight of 29.4 g. 295 mL of concentrated Bacillus cell suspension corresponds to a dry cell weight of 5.9 g; 490 mL of concentrated actinomycete cell suspension corresponds to a dry cell weight of 9.8 g. 1175 mL of concentrated Streptomyces cell suspension corresponds to a dry cell weight of 23.5 g.

[0040] The above bacterial suspension was added to a 15L fermenter and mixed, with a total dry weight of 100g. Sterile NBRIP liquid medium was added to the fermenter to bring the total volume to 10L. The mixture was cultured at 28℃, 120rpm, and an aeration rate of 0.5vvm for 60h to obtain phosphorus-solubilizing and phenolic carboxylic acid enrichment compound bacterial agent A.

[0041] Preparation Example 2 This preparation example provides a phosphorus-solubilizing and phenolic carboxylic acid enrichment functional compound bacterial agent, wherein the dry weight ratio of each bacterial species is as follows: Aspergillus niger : Penicillium oxalate : White rot fungi : Pseudomonas : Bacillus : Actinomycetes : Streptomyces = 15 : 10 : 3 : 18 : 6 : 12 : 14.

[0042] (1) Activation of each strain Take 1.0L of sterile NBRIP liquid culture medium and place it in a 3L Erlenmeyer flask with baffles. Inoculate each bacterial strain into the corresponding culture medium.

[0043] Aspergillus niger, Penicillium oxalate, and white rot fungi were cultured with shaking at 27℃ and 150 rpm for 60 h; Pseudomonas and Bacillus were cultured with shaking at 27℃ and 160 rpm for 48 h; and Actinomycetes and Streptomyces were cultured with shaking at 27℃ and 150 rpm for 60 h, respectively, to obtain activated cultures of each species.

[0044] (2) Collection of bacterial cells and adjustment of dry weight concentration Collect and wash the bacterial cells of each strain according to the method of Preparation Example 1, determine the dry weight concentration of the bacterial cells, and use sterile 0.85wt% sodium chloride aqueous solution to uniformly adjust the dry weight concentration of the concentrated bacterial cell suspension of each strain to 20g / L.

[0045] (3) Preparation of compound microbial agents Take 960 mL of concentrated Aspergillus niger cell suspension, corresponding to a cell dry weight of 19.2 g; 640 mL of concentrated Penicillium oxalate suspension corresponds to a dry weight of 12.8 g of cells; 190 mL of concentrated white-rot fungal suspension corresponds to a dry weight of 3.8 g of fungal cells; 1155 mL of concentrated Pseudomonas cell suspension corresponds to a dry cell weight of 23.1 g; 385 mL of concentrated Bacillus cell suspension corresponds to a dry cell weight of 7.7 g; 770 mL of concentrated actinomycete cell suspension corresponds to a dry cell weight of 15.4 g; 900 mL of concentrated Streptomyces cell suspension corresponds to a dry cell weight of 18.0 g.

[0046] The above bacterial suspension was added to a 15L fermenter and mixed, with a total dry weight of 100g. Sterile NBRIP liquid culture medium was added to the fermenter to bring the total volume to 10L. The mixture was cultured at 27℃, 130rpm, and an aeration rate of 0.5vvm for 60h to obtain phosphorus-solubilizing and phenolic carboxylic acid enrichment functional compound bacterial agent B.

[0047] Preparation Example 3 This preparation example provides a phosphorus-solubilizing and phenolic carboxylic acid enrichment functional compound bacterial agent, wherein the dry weight ratio of each bacterial species is as follows: Aspergillus niger : Penicillium oxalate : White rot fungi : Pseudomonas : Bacillus : Actinomycetes : Streptomyces = 20 : 15 : 5 : 20 : 10 : 18 : 15.

[0048] (1) Activation of each strain Take 1.0L of sterile NBRIP liquid culture medium and place it in a 3L Erlenmeyer flask with baffles. Inoculate each bacterial strain into the corresponding culture medium.

[0049] Aspergillus niger, Penicillium oxalate, and white rot fungi were cultured with shaking at 30℃ and 150 rpm for 72 h; Pseudomonas and Bacillus were cultured with shaking at 30℃ and 160 rpm for 48 h; and Actinomycetes and Streptomyces were cultured with shaking at 30℃ and 150 rpm for 72 h to obtain activated cultures of each species.

[0050] (2) Collection of bacterial cells and adjustment of dry weight concentration Collect and wash the bacterial cells of each strain according to the method of Preparation Example 1, determine the dry weight concentration of each bacterial cell, and adjust the dry weight concentration of the concentrated bacterial cell suspension of each strain to 20 g / L using a sterile 0.85 wt% sodium chloride aqueous solution.

[0051] (3) Preparation of compound microbial agents Take 970 mL of concentrated Aspergillus niger cell suspension, corresponding to a dry cell weight of 19.4 g; 730 mL of concentrated Penicillium oxalate suspension corresponds to a dry weight of 14.6 g of cells; 245 mL of concentrated white-rot fungal suspension corresponds to a dry weight of 4.9 g of fungal cells; 970 mL of concentrated Pseudomonas cell suspension corresponds to a dry cell weight of 19.4 g; 485 mL of concentrated Bacillus cell suspension corresponds to a dry cell weight of 9.7 g; 875 mL of concentrated actinomycete cell suspension corresponds to a dry cell weight of 17.5 g; 725 mL of concentrated Streptomyces cell suspension corresponds to a dry cell weight of 14.5 g.

[0052] The above bacterial suspension was added to a 15L fermenter and mixed, with a total dry weight of 100g. Sterile NBRIP liquid medium was added to the fermenter to bring the total volume to 10L. The mixture was cultured at 30℃, 120rpm, and an aeration rate of 0.6vvm for 72h to obtain the phosphorus-solubilizing and phenolic carboxylic acid enrichment compound bacterial agent C.

[0053] II. Plant Cultivation 1. Preparation of planting substrate The acidic red soil was air-dried naturally, and stones, plant roots, and other debris were removed. It was then crushed and sieved through a 5mm sieve. The pH of the acidic red soil used was 5.0. The organic fertilizer to be used was dried at 60℃ to constant weight. The mass fraction of the organic fertilizer in the planting substrate was calculated as the sum of the dry weight of the organic fertilizer and the dry weight of the acidic red soil.

[0054] The organic fertilizer was mixed evenly with the acidic red soil, and deionized water was added to bring the moisture content of the mixed substrate to 60% of its maximum water holding capacity. The mixture was then equilibrated at 23±2℃ for 5 days, turning it over daily during this period. The maximum water holding capacity was determined using the saturated drainage method: a known dry weight of substrate was fully saturated with water, allowed to drain for 24 hours, and then weighed. The maximum water holding capacity was calculated based on the mass of water remaining in the substrate after drainage and the mass of the dry substrate.

[0055] 2. Seedling selection, pre-culture and transplanting For *Dictamnus dasycarpus*, *Adiantum capillus-veneris*, and *Centipeda minima*, select seedlings without obvious diseases or pests, with intact leaves or pinnae, normal root development, and similar growth status. The relative average fresh weight of seedlings within the same treatment group should not deviate by more than ±10%.

[0056] Before transplanting, seedlings were pre-cultured for 7 days in acidic red soil without added phosphogypsum and organic fertilizer. During the pre-culture period, deionized water was added only on the first, third, and fifth days (each time the amount of water added was 20% of the weight of the acidic red soil), and no fertilizers containing rare earth elements were applied. Plastic planting pots with drainage holes at the bottom and an effective volume of 3L were used. Each pot was filled with planting substrate equivalent to 2.0 kg dry weight, and three plants of the same species were transplanted into each pot, evenly spaced along the circumference of the pot. During transplanting, the roots were allowed to spread naturally, with the root collar roughly level with the substrate surface. After transplanting, the substrate around the roots was gently pressed down. Five parallel planting pots were set up for each treatment group.

[0057] 3. Planting management and harvesting The plants were grown under conditions of 25℃, 60% relative humidity, and 13 hours of light per day. Planting pots from different treatment groups were randomly arranged and rotated every 3 days. The weight of the planting pots was checked daily, and deionized water was added to maintain the substrate moisture content at 65% ± 5% of maximum water holding capacity during the growing period. No additional phosphorus fertilizers, rare earth element fertilizers, or other conditioners that might introduce rare earth elements were applied during the growing period. The growing period was 30 days, calculated from the date of transplanting.

[0058] Thirty days after planting, the aboveground parts and roots of the plants were collected separately. First, the loose substrate adhering to the roots was shaken off, and the roots were rinsed with running water until no obvious soil or phosphogypsum particles were visible to the naked eye, followed by rinsing three times with deionized water. Then, the roots were immersed in a 10 mmol / L disodium ethylenediaminetetraacetate aqueous solution for 2 minutes, followed by rinsing three times with deionized water to reduce the influence of adsorbed particles on the root surface on the measurement results. The aboveground parts and roots were combined and dried at 70℃ to constant weight, and the dry weight of the plants was recorded.

[0059] III. Extraction methods of rare earth elements from phosphogypsum Unless otherwise stated, the amount of sodium carbonate added in the following examples is based on the dry weight of rare earth phosphogypsum; the mass ratio of biogas residue, thermally activated phosphogypsum and biomass additives is a dry weight ratio; the amount of functional compound microbial agent added is a percentage of the dry weight of the microbial cells in the compound microbial agent to the mass of volatile solids in the fermentation mixture; the "kg" in the air supply refers to one kilogram of wet mixture at the start of fermentation.

[0060] In Preparation Examples 1-3, the total dry weight concentration of the functional compound microbial agents was 10 g / L. To reduce the influence of moisture introduced by the inoculum on the moisture content of the fermentation mixture, the functional compound microbial agents were centrifuged at 5000×g for 10 min before use. The compound microbial cells were collected and resuspended in a sterile 0.85wt% sodium chloride aqueous solution to adjust the dry weight concentration of the microbial cells to 100 g / L, which was then used as the fermentation inoculum.

[0061] The method for determining the mass of volatile solids is as follows: take a sample of fermentation mixture, dry it at 105℃ to constant weight to obtain dry matter; ignite the dry matter at 550℃ to constant weight to obtain ash; the difference between the mass of dry matter and the mass of ash is the mass of volatile solids.

[0062] The wet basis weight of raw materials is calculated according to the following formula: Wet basis weight = Target dry basis weight ÷ (1 - Raw material moisture content).

[0063] Example 1

[0064] S1, thermal activation of rare earth phosphogypsum The rare earth phosphogypsum was dried at 60℃ to constant weight, crushed, and sieved through a 2mm sieve. 3500g of the dried rare earth phosphogypsum and 175g of anhydrous sodium carbonate were weighed, with the amount of anhydrous sodium carbonate added being 5% of the mass of the rare earth phosphogypsum.

[0065] Rare earth phosphogypsum and anhydrous sodium carbonate were added to a mixer and mixed at 200 rpm for 20 minutes. The resulting mixture was then evenly spread in a high-temperature resistant crucible to a thickness of 25 mm.

[0066] The crucible was placed in a muffle furnace and heated to 800°C at a rate of 5°C / min. After reaching 800°C, it was held at this temperature for 120 minutes to induce thermal decomposition of the carbonate-associated minerals in the rare-earth phosphogypsum and to restructure the mineral structure. After calcination, the crucible was cooled to below 200°C in the furnace, removed, and placed in a desiccator to cool to room temperature. The calcined product was crushed and sieved through a 1 mm sieve to obtain thermally activated phosphogypsum.

[0067] S2, Preparation of Ecological Organic Fertilizer The biogas residue used is the solid residue obtained from the anaerobic digestion and solid-liquid separation of livestock and poultry manure; the corn stalks are crushed to a length of less than 15 mm.

[0068] The moisture content of the biogas residue was measured to be 65 wt%, the moisture content of the thermally activated phosphogypsum was 2 wt%, and the moisture content of the corn stalks was 10 wt%.

[0069] Based on a dry basis mass ratio of 11:3:6 for biogas residue, thermally activated phosphogypsum, and corn stalks, weigh out 2.20 kg of biogas residue, 0.60 kg of thermally activated phosphogypsum, and 1.20 kg of corn stalks, respectively. The corresponding wet basis weighing amounts are 6.286 kg, 0.612 kg, and 1.333 kg, respectively.

[0070] The above raw materials were mixed evenly, and the volatile solids mass of the mixture was measured to be 2.80 kg. On the first day of aerobic fermentation, the phosphorus-solubilizing phenolic carboxylic acid enrichment functional compound microbial agent A obtained in Preparation Example 1 was added. The amount of functional compound microbial agent A added was 1% of the volatile solids mass of the mixture, corresponding to a required dry weight of 28.0 g of microbial cells.

[0071] The dry weight concentration of the inoculum of functional compound microbial agent A is 100 g / L, so 280 mL of inoculum is added. After adding functional compound microbial agent A, about 1.08 kg of deionized water is added and mixed thoroughly to adjust the moisture content of the mixture to 58 wt%, yielding 9.59 kg of wet-based fermentation mixture.

[0072] The fermentation mixture was loaded into an aerobic fermentation reactor with an effective volume of 30L, a height of 60cm, and a diameter of 40cm. The reactor was equipped with ventilation pipes and aeration holes at the bottom.

[0073] Continuous air supply is provided at a rate of 0.2 L / min per kilogram of initial wet mixture, with the total air supply controlled at 1.92 L / min. The material is turned over every 3 days to ensure uniform temperature, moisture content, and oxygen distribution in different parts of the material.

[0074] The moisture content of the mixture was measured every 2 days during fermentation. When the moisture content was below 55 wt%, deionized water was sprayed evenly to restore the moisture content of the mixture to 55-60 wt%. When the local material temperature exceeded 70℃, the material was turned over or the air supply was increased for a short period of time to cool it down.

[0075] The total aerobic fermentation time was 42 days, yielding an ecological organic fertilizer containing coumaric acid, salicylic acid, ferulic acid, and caffeic acid.

[0076] S3, Dicranopteris planting and rare earth element enrichment The organic fertilizer obtained in step S2 is dried at 60℃ to constant weight and mixed with acidic red soil at a ratio of 20% of the dry weight of the organic fertilizer to the total dry weight of the planting substrate.

[0077] The total dry weight of the planting substrate for each pot is 2.0 kg, of which the dry weight of the organic fertilizer is 0.40 kg and the dry weight of the acidic red soil is 1.60 kg.

[0078] Following the aforementioned planting method, the organic fertilizer was mixed evenly with the acidic red soil, and the moisture content was adjusted and balanced for 5 days. Three plants of uniform growth were transplanted into each pot, and five parallel planting pots were set up for each treatment group.

[0079] The plants were grown for 30 days under conditions of 25℃ temperature, 60% relative humidity, and 13 hours of light per day. During the growing period, the moisture content of the planting substrate was maintained at 65% ± 5% of the maximum water holding capacity, allowing the sedge to absorb and enrich rare earth elements from rare earth phosphogypsum.

[0080] S4, plant ashing and rare earth oxide recovery Thirty days after planting, the above-ground parts and roots of the sedge were collected, and the planting substrate particles attached to the plant surface were removed according to the aforementioned plant harvesting and cleaning methods. The above-ground parts and roots were combined, dried at 70°C to constant weight, and cut to a length not exceeding 10 mm.

[0081] The dried plants were placed in a porcelain crucible that had been preheated to constant weight. The temperature was increased to 300°C at a rate of 5°C / min, and the mixture was held at this temperature for 30 min to pre-carbonize. The temperature was then increased to 600°C at a rate of 5°C / min, and the mixture was held at this temperature for 30 min to ashed. After cooling, the plant ash was obtained.

[0082] Grind 10.00g of plant ash and pass it through a 0.25mm sieve. Acid leaching is performed using 200mL of 1mol / L hydrochloric acid at a liquid-to-solid ratio of 1g:20mL. The hydrochloric acid is added to the plant ash in three equal portions, with stirring at 300rpm for 20min after each addition. After all the hydrochloric acid has been added, leaching continues at 40℃ and 300rpm for 60min.

[0083] After acid leaching, the solution was filtered using medium-speed filter paper. The residue was washed twice with a small amount of deionized water. The filtrate and washings were combined to obtain a rare earth-containing leachate. Under stirring at 300 rpm, a 10 wt% sodium hydroxide aqueous solution was slowly added dropwise to adjust the pH of the rare earth-containing leachate to 5.0, and stirring was continued for 20 minutes.

[0084] 1.0 mL of 10 wt% ammonium oxalate aqueous solution was slowly added dropwise to the rare earth-containing leachate over a period of 5 min. After the addition was complete, the mixture was stirred at 300 rpm for 45 min, then allowed to stand for 2 h. The mixture was then filtered to obtain rare earth oxalate precipitate. The precipitate was washed three times with deionized water and dried at 105 °C to constant weight. Subsequently, the temperature was increased to 850 °C at a rate of 5 °C / min, and calcined for 2 h after reaching 850 °C to obtain rare earth oxides.

[0085] Example 2

[0086] S1, thermal activation of rare earth phosphogypsum Rare earth phosphogypsum (composed of 20.41 wt% fluorapatite, 1.07 wt% quartz, and 78.52 wt% dolomite) was dried to constant weight at 60°C, crushed, and passed through a 2 mm sieve. 3500 g of the dried rare earth phosphogypsum and 140 g of anhydrous sodium carbonate were weighed out, with the amount of anhydrous sodium carbonate added being 4% of the mass of the rare earth phosphogypsum.

[0087] Rare earth phosphogypsum and anhydrous sodium carbonate were mixed at 200 rpm for 20 min. The resulting mixture was then evenly spread in a high-temperature resistant crucible with a thickness of 25 mm.

[0088] The resulting mixture was placed in a high-temperature resistant crucible and heated to 850°C at a rate of 5°C / min. After reaching 850°C, the temperature was maintained for calcination for 100 min. After calcination, the mixture was cooled to below 200°C in the furnace and then placed in a desiccator to cool to room temperature. The calcined product was crushed and passed through a 1 mm sieve to obtain thermally activated phosphogypsum.

[0089] S2, Preparation of Ecological Organic Fertilizer The biogas residue used is the solid residue obtained from the anaerobic digestion and solid-liquid separation of livestock and poultry manure; rice husks are crushed to a length of less than 15 mm. The moisture content of the biogas residue was determined to be 65 wt%, the moisture content of the thermally activated phosphogypsum was 2 wt%, and the moisture content of the rice husks was 8 wt%.

[0090] Based on a dry basis mass ratio of 13:4:6.5 for biogas residue, thermally activated phosphogypsum, and rice husk, weigh out 2.60 kg of biogas residue, 0.80 kg of thermally activated phosphogypsum, and 1.30 kg of rice husk, respectively. The corresponding wet basis weighing amounts are 7.429 kg, 0.816 kg, and 1.413 kg, respectively.

[0091] The above raw materials were mixed evenly, and the volatile solids mass of the mixture was measured to be 3.25 kg. On the first day of fermentation, the functional compound microbial agent B obtained in Preparation Example 2 was added. The amount of functional compound microbial agent B added was 3% of the volatile solids mass of the mixture, corresponding to a required dry weight of 97.5 g of microbial cells.

[0092] The dry weight concentration of the inoculum in the functional compound microbial agent B is 100 g / L, so 975 mL of inoculum was added. After adding the inoculum, about 0.03 kg of deionized water was added to adjust the moisture content of the mixture to 55 wt%, resulting in 10.66 kg of wet-based fermentation mixture.

[0093] The fermentation mixture was loaded into an aerobic fermentation reactor with an effective volume of 30L, a height of 60cm, and a diameter of 40cm. The reactor was equipped with ventilation pipes and aeration holes at the bottom.

[0094] Continuous air supply is provided at a rate of 0.1 L / min per kilogram of initial wet base mixture, with the total air supply controlled at 1.07 L / min. The material is turned over every 2 days, and deionized water is added according to the moisture content measurement results to maintain the moisture content at 55-60 wt% during fermentation. When the local material temperature exceeds 65℃, cooling is achieved by turning the material over or increasing the air supply for a short period.

[0095] The total aerobic fermentation time was 35 days, yielding an ecological organic fertilizer containing coumaric acid, salicylic acid, ferulic acid, and caffeic acid.

[0096] S3, cultivation of black fern and enrichment of rare earth elements The organic fertilizer obtained in step S2 was applied to the acidic red soil at a dry basis weight fraction of 18% (the main components were kaolinite 34.8 wt%, hematite 15.7 wt%, and quartz 32.4 wt%, sourced from Kunming, Yunnan). The total dry basis weight of each potting substrate was 2.0 kg, of which the dry basis weight of the organic fertilizer was 0.36 kg and the dry basis weight of the acidic red soil was 1.64 kg.

[0097] Following the aforementioned planting method, the organic fertilizer was mixed evenly with the acidic red soil, and the moisture content was adjusted and balanced for 5 days. Three ferns of uniform growth were transplanted into each pot, and five parallel planting pots were set up for each treatment group.

[0098] The plants were grown for 30 days under conditions of 25℃, 60% relative humidity, and 13 hours of light per day. During the growing period, the moisture content of the growing substrate was maintained at 65% ± 5% of the maximum water holding capacity, allowing the *Cereus undatus* to absorb and enrich rare earth elements from rare earth phosphogypsum.

[0099] S4, plant ashing and rare earth oxide recovery The above-ground parts and roots of *Cereus undatus* were collected after 30 days of cultivation, washed, and dried at 70℃ to constant weight. The dried parts were then ashed at 600℃ for 30 minutes to obtain plant ash.

[0100] Take 10.00g of plant ash and add 200mL of 1mol / L hydrochloric acid at a liquid-solid ratio of 1g:20mL for acid leaching. The leaching temperature is 40℃, the stirring speed is 300rpm, and the leaching time is 60min. After acid leaching, filter and wash the residue twice with a small amount of deionized water. Combine the filtrate and washings to obtain a rare earth-containing leachate.

[0101] The pH of the rare earth-containing leachate was adjusted to 4.5 using a 10wt% sodium hydroxide aqueous solution, and stirring was continued for 20 min. 1.0 mL of a 10wt% ammonium oxalate aqueous solution was slowly added dropwise to the rare earth-containing leachate over a period of 5 min. After the addition was completed, stirring was continued for 45 min, and the mixture was allowed to stand for 2 h. The rare earth oxalate precipitate was obtained by filtration.

[0102] The rare earth oxalate precipitate was washed three times with deionized water and dried at 105℃ to constant weight. Then, the temperature was increased to 800℃ at 5℃ / min and calcined for 1 hour after reaching 800℃ to obtain rare earth oxides.

[0103] Example 3

[0104] S1, thermal activation of rare earth phosphogypsum The rare earth phosphogypsum was dried at 60°C to constant weight, crushed, and sieved through a 2mm sieve. 3500g of the dried rare earth phosphogypsum and 210g of anhydrous sodium carbonate were weighed, with the amount of anhydrous sodium carbonate added being 6% of the mass of the rare earth phosphogypsum.

[0105] Rare earth phosphogypsum and anhydrous sodium carbonate were mixed at 200 rpm for 20 min. The resulting mixture was then evenly spread in a high-temperature resistant crucible with a thickness of 25 mm.

[0106] The resulting mixture was placed in a high-temperature resistant crucible and heated to 900°C at a rate of 5°C / min. After reaching 900°C, the mixture was calcined for 150 min. After calcination, the mixture was cooled to below 200°C in the furnace and then placed in a desiccator to cool to room temperature. The calcined product was crushed and passed through a 1 mm sieve to obtain thermally activated phosphogypsum.

[0107] S2, Preparation of Ecological Organic Fertilizer The biogas residue used is the solid residue obtained from the anaerobic digestion and solid-liquid separation of livestock and poultry manure; corn stalks and rice husks are crushed to a length of less than 15 mm. The corn stalks, rice husks and landscaping waste are mixed at a dry basis mass ratio of 1:1:1 to obtain biomass feedstock.

[0108] The moisture content of biogas residue was measured to be 65 wt%, that of thermally activated phosphogypsum was 2 wt%, that of corn stalks was 10 wt%, that of rice husks was 8 wt%, and that of garden waste was 15 wt%.

[0109] Based on a dry weight ratio of 15:5:7 for biogas residue, thermally activated phosphogypsum, and biomass additives, weigh out 3.00 kg of biogas residue, 1.00 kg of thermally activated phosphogypsum, and 1.40 kg of biomass additives, respectively.

[0110] The 1.40 kg dry-based biomass additives include 0.467 kg of corn stalks, 0.467 kg of rice husks, and 0.466 kg of landscaping waste. The corresponding wet-based quantities are approximately 0.519 kg, 0.508 kg, and 0.548 kg, respectively. The corresponding wet-based quantities of biogas residue and thermally activated phosphogypsum are 8.571 kg and 1.020 kg, respectively.

[0111] The above raw materials were mixed evenly, and the volatile solids mass of the mixture was measured to be 3.75 kg. On the first day of fermentation, the functional compound microbial agent C obtained in Preparation Example 3 was added. The amount of functional compound microbial agent C added was 5% of the volatile solids mass of the mixture, corresponding to a required dry weight of 187.5 g of microbial cells.

[0112] The dry weight concentration of the inoculum of functional compound microbial agent C is 100 g / L, therefore 1.875 L of inoculum was added. After adding the inoculum, 0.93 kg of deionized water was added to adjust the moisture content of the mixture to 60 wt%, yielding 13.97 kg of wet-based fermentation mixture.

[0113] The fermentation mixture was loaded into a 30L aerobic fermentation reactor, and air was continuously supplied at a rate of 0.3L / min per kilogram of initial wet base mixture, with the total air supply controlled at approximately 4.19L / min. The material was turned over every 3 days, and deionized water was added to maintain the moisture content at 55-60 wt% during the fermentation process.

[0114] The total aerobic fermentation time was 49 days, yielding an ecological organic fertilizer containing coumaric acid, salicylic acid, ferulic acid, and caffeic acid.

[0115] S3, Centipede Grass Cultivation and Rare Earth Element Enrichment The organic fertilizer obtained in step S2 is applied to the acidic red soil at a dry basis weight fraction of 25%. The total dry basis weight of each potting substrate is 2.0 kg, of which the dry basis weight of the organic fertilizer is 0.50 kg and the dry basis weight of the acidic red soil is 1.50 kg.

[0116] Following the aforementioned planting method, the organic fertilizer was mixed evenly with the acidic red soil, and the moisture content was adjusted and balanced for 5 days. Three centipede grass plants with the same growth status were transplanted into each pot, and five parallel planting pots were set up for each treatment group.

[0117] The plants were grown for 30 days under conditions of 25℃, 60% relative humidity, and 13 hours of light per day. During the growing period, the moisture content of the growing substrate was maintained at 65% ± 5% of the maximum water holding capacity, allowing the centipede grass to absorb and enrich rare earth elements from rare earth phosphogypsum.

[0118] S4, plant ashing and rare earth oxide recovery The upper part and roots of the centipede grass were collected after 30 days of planting, washed, and dried at 70℃ to constant weight. After reaching 600℃, the mixture was kept at that temperature for 30 minutes to obtain plant ash.

[0119] Take 10.00g of plant ash and add 200mL of 1mol / L hydrochloric acid at a liquid-solid ratio of 1g:20mL for acid leaching. The leaching temperature is 40℃, the stirring speed is 300rpm, and the leaching time is 60min. After acid leaching, filter and wash the residue twice with a small amount of deionized water. Combine the filtrate and washings to obtain a rare earth-containing leachate.

[0120] The pH of the rare earth-containing leachate was adjusted to 5.5 using a 10wt% sodium hydroxide aqueous solution, and stirring was continued for 20 min. 1.0 mL of a 10wt% ammonium oxalate aqueous solution was slowly added dropwise to the rare earth-containing leachate over a period of 5 min. After the addition was completed, stirring was continued for 45 min, and the mixture was allowed to stand for 2 h. The rare earth oxalate precipitate was obtained by filtration.

[0121] The rare earth oxalate precipitate was washed three times with deionized water and dried at 105℃ to constant weight. Then, the temperature was increased to 900℃ at 5℃ / min and calcined for 3 hours after reaching 900℃ to obtain rare earth oxides.

[0122] The parameters not specified in Examples 2 and 3 above are the same as those in Example 1.

[0123] control group The method for extracting rare earth elements from phosphogypsum is basically the same as in Example 2, except that step S2 is not involved; in step S3, the ecological organic fertilizer is replaced with phosphogypsum of equal dry weight.

[0124] Comparative Example 1 This comparative example is basically the same as Example 2, except that: in step S1, the rare earth phosphogypsum is directly calcined at 850°C for 120 minutes without adding sodium carbonate; the remaining fermentation, sedge planting, plant ashing, hydrochloric acid leaching, ammonium oxalate precipitation and calcination conditions are the same as in Example 1.

[0125] Comparative Example 2 This comparative example is basically the same as Example 2, except that in step S1, the calcination temperature is reduced from 850°C to 700°C, while the calcination time remains 100 min; all other conditions are the same as in Example 1.

[0126] Comparative Example 3 This comparative example is basically the same as Example 2, except that in step S2, no phosphorus-solubilizing phenolic carboxylic acid enrichment compound microbial agent is added in the initial stage of aerobic fermentation, and natural aerobic fermentation is carried out only by relying on the original microorganisms in the biogas residue and biomass auxiliary materials; the other conditions are the same as in Example 1.

[0127] Comparative Example 4 This comparative example is basically the same as Example 2, except that in step S2, the functional compound microbial agent B is replaced with a simplified microbial agent of equal dry weight. The simplified microbial agent is composed only of Aspergillus niger, Penicillium oxalate, Pseudomonas, and Bacillus in a dry weight ratio of 10:5:15:3, and does not contain white-rot fungi, actinomycetes, or Streptomyces.

[0128] Comparative Example 5 This comparative example is basically the same as Example 1, except that in step S3, the sedge is replaced with perennial ryegrass with similar growth status, 3 plants are planted in each pot, and the planting time is still 30 days; the other planting conditions, as well as the plant ashing and rare earth recovery conditions, are the same as in Example 1.

[0129] Table 2 shows the content indicators of rare earth oxides prepared and extracted in Examples 1-3, the control group, and Comparative Examples 1-5.

[0130] Table 2

[0131] As can be seen from the data in the examples and comparative example 1 in the table, under the same roasting temperature and subsequent treatment conditions, when sodium carbonate was not added during roasting, the converted content of rare earth oxides in plant ash and the purity of rare earth oxides in calcined products both decreased. This indicates that sodium carbonate can synergistically promote the thermal decomposition and mineral phase reconstruction of phosphogypsum associated minerals with high-temperature roasting, which is beneficial to the exposure of rare earth occurrence phases and subsequent migration and release.

[0132] Comparative Example 2, when the roasting temperature was reduced to 700℃, showed a further decrease in the converted content of rare earth oxides in the plant ash and the purity of rare earth oxides in the calcined products. This indicates that the lower roasting temperature has insufficient destructive effect on the mineral crystal encapsulation structure, and that roasting conditions of 800-900℃ are conducive to the formation of mineral structures suitable for microbial activation and plant absorption.

[0133] Comparative Example 3 did not add the phosphorus-solubilizing phenolic carboxylic acid enrichment functional compound microbial agent, and relied solely on the original microorganisms in the raw materials for natural fermentation. Both of its indicators were significantly lower than those of Example 1, indicating that the functional compound microbial agent can promote the activation of insoluble phosphorus, the migration of rare earth elements, and the formation of low molecular weight aromatic phenolic carboxylic acids.

[0134] Comparative Example 4, using only a phosphate-solubilizing bacterial agent composed of Aspergillus niger, Penicillium oxalate, Pseudomonas, and Bacillus, showed higher performance than Comparative Example 3 (which did not include a bacterial agent), but lower performance than Example 1, which used a complete functional compound bacterial agent. These results indicate that the low-molecular-weight organic acids produced by phosphate-solubilizing bacteria can promote the migration of phosphorus and rare earth elements. Furthermore, the participation of white-rot fungi, actinomycetes, and Streptomyces in biomass conversion and the promotion of aromatic phenolic carboxylic acid formation can further improve the rhizosphere environment and plant enrichment effects.

[0135] In Comparative Example 5, after replacing *Dicranopteris dichotoma* with ryegrass, the converted content of rare earth oxides in the plant ash decreased significantly, indicating that plant species have a significant impact on the transfer of rare earth elements from the planting substrate to the plant body. Using *Dicranopteris dichotoma*, *Dryopteris crassirhizoma*, or *Centipeda minima* is beneficial to improving the plant enrichment of rare earth elements.

[0136] IV. Test Analysis 1. Analysis of low molecular weight aromatic phenolic carboxylic acids Thermal activation of rare earth phosphogypsum: The rare earth phosphogypsum was dried at 60℃ to constant weight, crushed, and passed through a 2mm sieve. 3500g of the dried rare earth phosphogypsum and 140g of anhydrous sodium carbonate were weighed, with the amount of anhydrous sodium carbonate added being 4% of the mass of the rare earth phosphogypsum.

[0137] Rare earth phosphogypsum and anhydrous sodium carbonate were mixed at 200 rpm for 20 min. The resulting mixture was then evenly spread in a high-temperature resistant crucible with a thickness of 25 mm.

[0138] The resulting mixture was placed in a high-temperature resistant crucible and heated to 200 / 500 / 700 / 800 / 900℃ at a rate of 5℃ / min, and calcined for 100 min. After calcination, the mixture was cooled to below 200℃ in the furnace and then placed in a desiccator to cool to room temperature. The calcined product was crushed and passed through a 1 mm sieve to obtain thermally activated phosphogypsum.

[0139] The thermally activated phosphogypsum treated at different calcination temperatures was processed according to the fermentation conditions of Example 2. The low-molecular-weight aromatic phenolic carboxylic acids in the fermentation products were detected, and the results are as follows: Figure 1 As shown, with increasing roasting temperature, especially above 700℃, the relative enrichment of salicylic acid, ferulic acid, and caffeic acid generally increases, indicating that the rough surface and active interface formed by high-temperature roasting are conducive to the accumulation of fermentation products and aromatic phenolic carboxylic acids on the mineral surface.

[0140] 2. ICP-MS elemental composition Following the process described in Example 2, the elemental content of the thermally activated phosphogypsum mixed acid obtained at different calcination temperatures was analyzed after complete digestion. The test results are shown in Table 3.

[0141] Table 3

[0142] As shown in Table 3, the ΣREE+Y content in the unroasted sample was 503.08 mg / kg, in the 500℃ roasted sample it was 454.25 mg / kg, and in the 850℃ roasted sample it was 617.60 mg / kg. The detected contents of Y, La, Nd, and Ce in the 850℃ roasted sample were 212.40 mg / kg, 134.63 mg / kg, 95.87 mg / kg, and 70.23 mg / kg, respectively. Compared with the unroasted and 500℃ roasted samples, the 850℃ roasted sample showed higher levels of ΣREE+Y and various major rare earth elements, indicating that high-temperature roasting is beneficial for exposing the rare earth host phase and the relative enrichment of rare earth components. The 500℃ roasted sample contained fewer rare earth elements than the unroasted sample, mainly because this temperature is insufficient to completely decompose and transform the rare earth minerals, resulting in rare earth elements remaining in the insoluble phase or being secondary encapsulated, leading to a lower subsequent recovery rate.

[0143] 3. XRD phase analysis Following the process described in Example 2, rare earth oxides obtained from phosphogypsum calcination at different temperatures were subjected to XRD analysis. The test results are shown in Table 4.

[0144] Table 4

[0145] Note: The relative content of XRD is a semi-quantitative result of material matching, which is only used to illustrate the trend of mineral phase change at different roasting temperatures and is not equivalent to the absolute content of elements in the sample.

[0146] Figure 2-4 The images show the XRD patterns of uncalcined phosphogypsum, thermally activated phosphogypsum obtained by calcining phosphogypsum at 500℃, and thermally activated phosphogypsum obtained by calcining phosphogypsum at 850℃, respectively.

[0147] The unroasted sample mainly contained fluorapatite, dolomite, and a small amount of quartz. After roasting at 500℃, the main phases were basically the same as those in the unroasted sample, indicating that the 500℃ treatment had limited effect on the destruction of mineral structure and phase transformation. After roasting at 850℃, the characteristic peaks of dolomite were significantly weakened or disappeared, and new phases such as MgO, CaO, calcite, aragonite, and Ca(OH)2 appeared, indicating that high-temperature roasting promoted the thermal decomposition and mineral phase reconstruction of associated carbonate minerals and increased the relative exposure of the apatite phase.

[0148] 4. SEM-EDS Analysis Figure 5The images show the SEM morphology of phosphogypsum at different calcination temperatures. Uncalcined samples mainly exhibit irregular blocky and fragmented particles with relatively intact edges and a dense surface. After calcination at 500℃, the particle edges begin to break, and the surface roughness increases. After calcination at 850℃, particle breakage and surface roughness become more pronounced, with visible microcracks and a loose structure. These morphological changes indicate that calcination at 850℃ increases the particle reaction interface, providing favorable conditions for microbial organic acid action, rare earth element migration and release, and plant uptake. Table 5 shows the rare earth element signals at representative EDS spectral points at different calcination temperatures.

[0149] Table 5

[0150] Note: EDS results are semi-quantitative results of local spectral points, which are only used to illustrate that rare earth element signals and their exposure characteristics can be detected locally in sample particles, and do not represent the overall rare earth content of the sample; the overall rare earth abundance of the sample shall be based on the ICP-MS results.

[0151] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. However, any modifications, alterations, and variations made by those skilled in the art without departing from the scope of the present invention based on the disclosed technical content are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A method for extracting rare earth elements from phosphogypsum, characterized in that, Includes the following steps: S1. Rare earth phosphogypsum is mixed with sodium carbonate and calcined to cause thermal decomposition and mineral phase reconstruction of the associated minerals in the phosphogypsum, resulting in thermally activated phosphogypsum. S2. Mix biogas residue, the thermally activated phosphogypsum and biomass additives, adjust the moisture content of the mixture, add phosphorus-solubilizing phenolic carboxylic acid enrichment functional compound microbial agent at the initial stage of aerobic fermentation and carry out aerobic fermentation to obtain ecological organic fertilizer containing low molecular weight aromatic phenolic carboxylic acid. S3. Apply the ecological organic fertilizer to the planting substrate and plant plants, so that the plants can absorb and enrich rare earth elements from phosphogypsum. S4. Collect the plants obtained in step S3, dry and ashed to obtain plant ash; acid leaching of the plant ash to separate rare earth-containing leachate, and oxalate precipitation and calcination of the rare earth-containing leachate to obtain rare earth oxides.

2. The method according to claim 1, characterized in that, In step S1, the amount of sodium carbonate added is 4-6% of the mass of rare earth phosphogypsum, the calcination temperature is 800-900℃, and the calcination time is 100-150min.

3. The method according to claim 1, characterized in that, In step S2, the mass ratio of the biogas residue, thermally activated phosphogypsum and biomass additives is (11-15):(3-5):(6-7), and the moisture content of the mixture is 55-60 wt%.

4. The method according to claim 1, characterized in that, The biomass feedstock is selected from one or more of corn stalks, rice husks, and garden waste.

5. The method according to claim 1, characterized in that, The phosphorus-solubilizing-phenolic carboxylic acid enrichment functional compound microbial agent comprises, by dry weight of microorganisms, 10-20 parts of Aspergillus niger, 5-15 parts of Penicillium oxalate, 1-5 parts of white-rot fungi, 15-20 parts of Pseudomonas, 3-10 parts of Bacillus, 5-18 parts of Actinomycetes and 12-15 parts of Streptomyces.

6. The method according to claim 5, characterized in that, The preparation method of the phosphorus-solubilizing-phenolic carboxylic acid enrichment functional compound bacterial agent includes: activating each bacterial strain separately in phosphorus-solubilizing medium, mixing them, and culturing at 25-30℃ for 48-72h.

7. The method according to claim 1, characterized in that, In step S2, the amount of the functional compound microbial agent added is 1-5% of the volatile solids mass of the mixture; the air supply for aerobic fermentation is 0.1-0.3 L / min / kg, the temperature during the high-temperature period is greater than 55℃ and maintained for at least 5 days, and the total fermentation time is 35-49 days.

8. The method according to claim 1, characterized in that, In step S3, the plant with rare earth enrichment ability is selected from one or more of the following: *Dictamnus dasycarpus*, *Adiantum capillus-veneris*, and *Centipeda minima*; the ecological organic fertilizer has a mass fraction of 18-25% in the planting substrate, and the planting time is 30 days.

9. The method according to claim 1, characterized in that, In step S4, the ashing is carried out at 600℃ for 30 minutes; the acid leaching is carried out with 1 mol / L hydrochloric acid, and the solid-liquid separation is performed after acid leaching to obtain the rare earth-containing leachate.

10. The method according to claim 9, characterized in that, The pH of the rare earth-containing leachate is adjusted to 4.5-5.5, and ammonium oxalate is added to form rare earth oxalate precipitate. The rare earth oxalate precipitate is washed, dried, and then calcined at 800-900℃ for 1-3 hours to obtain the rare earth oxide.

Images