A harmless treatment process based on rare earth calcium slag green residue

Abstract

This invention discloses a green and harmless treatment process for rare earth calcium slag, belonging to the field of rare earth metal extraction and waste treatment technology. This process aims to solve the problems of heavy environmental pollution, high energy consumption, and poor selectivity in existing technologies. Its core lies in: providing a bifunctional complexing agent containing ion-capturing and ion-affinity ends (e.g.); dissolving it in supercritical carbon dioxide fluid to form an extraction medium; contacting this medium with pretreated rare earth calcium slag at 40-100°C and 10-30 MPa to selectively extract rare earth elements and form rare earth-complexes; finally, separating the rare earth-complexes by depressurization and recycling carbon dioxide and the complexing agent. This process achieves a rare earth recovery rate >92%, product purity >99%, complexing agent and single-cycle utilization rate >98%, and the fluoride leaching concentration in the treated residue is lower than the national standard limit, achieving harmlessness. It has significant advantages such as a green and environmentally friendly process, low energy consumption, high selectivity, and simplified process.

Description

Technical Field

[0001] This invention belongs to the field of rare earth metal acquisition and waste treatment technology, specifically, it relates to a green slag harmless treatment process based on rare earth calcium slag. Background Technology

[0002] Rare earth calcium slag is the main solid waste generated during the metallothermic reduction process for rare earth metals. Its main component is chemically stable calcium fluoride. Rare earth calcium slag contains highly alkaline calcium oxide (CaO) and physically encapsulated valuable rare earth metals (RE). If left untreated, this slag not only results in the loss of valuable rare earth resources but also poses a long-term threat to the environment due to its radioactivity. Therefore, developing an advanced process capable of harmlessly treating rare earth calcium slag and efficiently recovering rare earth resources has become an urgent technological need.

[0003] Traditional rare earth element (REE) treatment processes are divided into wet and pyrometallurgical methods. Wet processes use strong acids (such as sulfuric acid and hydrochloric acid) for leaching, which can dissolve some REEs but also release large amounts of fluorides and calcium salts, producing high-salt, fluoride-containing acidic wastewater. Subsequent treatment is costly and prone to secondary pollution. Pyrometallurgical processes achieve REE enrichment through high-temperature treatment above 1400℃, but they are energy-intensive, require demanding equipment, and may produce harmful fumes. Dry processes such as medium-temperature chlorination volatilization, proposed in recent years, offer relatively milder conditions, but consume large amounts of reagents and require highly corrosion-resistant and airtight equipment, which limits their economic viability and safety for large-scale industrial application.

[0004] Traditional processes rely on the macroscopic effects of high-intensity media on rare earth calcium slag, lacking selective separation of rare earth elements, thus exhibiting bottlenecks under environmental protection and resource-efficient utilization requirements. Essentially, they overcome the inertia of the slag by sacrificing environmental or energy resources, failing to meet the demands of sustainable development. How to selectively extract rare earth elements from the slag under mild conditions, avoiding secondary pollution, has become a fundamental challenge.

[0005] Therefore, a disruptive technology is urgently needed to achieve highly selective separation of rare earth elements under non-aqueous and non-high-temperature conditions. Supercritical fluid technology has shown potential due to its tunable physicochemical properties (such as density and viscosity), but how to design and implement a mechanism that enables efficient and highly selective "dissolution" and "transport" of specific metal ions in an inert solid phase in a non-polar or weakly polar environment of supercritical fluid, while ensuring the economic efficiency and environmental friendliness of the process, remains a key challenge and a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a novel, environmentally friendly, near-room temperature-operated, green and harmless treatment process based on rare earth calcium slag. This process utilizes a supercritical fluid-complexing agent synergistic system to achieve highly selective extraction and separation of rare earth elements.

[0007] To achieve the above-mentioned objectives, the present invention employs the following technical solution: A green and harmless treatment process for rare earth calcium slag includes the following steps: (a) Pretreatment: The rare earth calcium slag is mechanically crushed to obtain particles with an average particle size of 20 micrometers to 200 micrometers, and the particles are dried to remove moisture. (b) Providing a bifunctional complexing agent: the bifunctional complexing agent comprises an ion-trapping terminal for selectively chelating rare earth ions and a hydrophilic terminal for improving their solubility in supercritical carbon dioxide. end; (c) Forming an extraction medium: The bifunctional complexing agent is dissolved in supercritical carbon dioxide fluid to form an extraction medium, wherein the concentration of the bifunctional complexing agent is in the range of 0.01 mol / L to 0.2 mol / L; (d) Extraction contact: Under extraction conditions of 40°C to 100°C and 10 MPa to 30 MPa, the extraction medium is contacted with the pretreated rare earth calcium slag particles, so that the bifunctional complexing agent reacts with the rare earth elements to generate rare earth-complexes and dissolves in supercritical carbon dioxide fluid. (e) Separation and recovery: The supercritical carbon dioxide fluid loaded with the rare earth-complex is subjected to depressurization to reduce its pressure below the critical pressure, thereby causing the rare earth-complex to precipitate and be separated and collected.

[0008] Further, the drying conditions described in step (a) are: heating at 100 to 150 degrees Celsius for 2 to 6 hours, or drying under vacuum conditions.

[0009] Further, the ion-trapping terminal in step (b) comprises a group selected from phosphate esters, phosphonates, carboxylic acids, etc. At least one functional group selected from diketone, hydroxyl, and amino groups; the affinity The terminal contains at least one functional group selected from perfluorinated alkyl chains, fluorinated ether chains, polysiloxane chains, or highly branched saturated hydrocarbon chains.

[0010] Furthermore, the ion-trapping terminal is a functional group containing a phosphate ester group or a phosphonic acid group; the hydrophilic... The terminal is a perfluoroalkyl chain with 6-20 carbon atoms.

[0011] Furthermore, the aforementioned parent A perfluoroalkyl chain with 8-12 carbon atoms at the end.

[0012] Further, the extraction conditions described in step (d) are: temperature 60 degrees Celsius and pressure 20 MPa.

[0013] Furthermore, step (e) is followed by step (f) complexing agent regeneration: the separated and collected rare earth-complex is dissociated to release rare earth ions and regenerate the bifunctional complexing agent, which is then returned to step (c) for recycling.

[0014] Furthermore, the dissociation treatment is achieved by washing with a weakly acidic aqueous solution with a pH of 3-5.

[0015] Furthermore, step (e) is followed by step (g) carbon dioxide recycling: the gaseous carbon dioxide after the rare earth-complex is separated is compressed and purified and reused to form the extraction medium described in step (c).

[0016] Beneficial effects: (1) This invention uses non-toxic, non-flammable, and low-cost supercritical carbon dioxide as the core solvent. The entire extraction and separation process is carried out in a mild chemical environment free of water, strong acids / bases, thus eliminating the generation of large amounts of acidic wastewater and fluoride-containing waste residue in traditional wet processes. The core reagent (bifunctional complexing agent) and solvent ( All of these technologies can achieve efficient recycling and closed-loop circulation (single-cycle utilization rate > 98%), and the emissions of waste gas, wastewater, and solid waste from the entire process are close to zero. It is a green and clean production technology with significant environmental benefits.

[0017] (2) The operating temperature range of the core steps of the process of this invention is 35-80°C (near room temperature), and the operating pressure is 10-30MPa (medium pressure), which is in stark contrast to the extreme high temperature of traditional pyrometallurgy, which often exceeds 1,000 degrees Celsius, and the high energy consumption of high-temperature molten salt electrolysis. This not only significantly reduces energy consumption, but also reduces the requirements for high-temperature resistant and corrosion-resistant special materials for the reactor, significantly reducing equipment investment and maintenance costs, and has outstanding economic benefits.

[0018] (3) This invention utilizes the specific recognition and coordination capabilities of bifunctional complexing agents, enabling their ion-capturing ends to bind with rare earth ions efficiently and selectively, while showing almost no reaction to matrix components such as calcium and fluorine. This molecular-level selectivity fundamentally avoids the co-extraction of impurities, resulting in rare earth intermediates with a purity of up to 99.0%, greatly simplifying the purification process for preparing high-purity rare earth oxides or metals and reducing overall costs.

[0019] (4) This invention utilizes the characteristics of supercritical fluids, and the product separation step can be achieved through simple physical decompression. It rapidly transforms into a gaseous state, and the dissolved rare earth complex instantly precipitates as a high-purity solid. This process is fast and thorough, eliminating the need for multiple chemical separation steps such as evaporation, crystallization, and back-extraction found in traditional processes. This simplifies the process, improves product recovery efficiency, and facilitates solvent recycling. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of this invention, the following will provide a more detailed description of this application in conjunction with embodiments. Unless otherwise specified, the reagents, equipment, and methods used in this application are conventional choices in the field.

[0021] Examples and synthesis of bifunctional complexing agents: The bifunctional complexing agent must possess both ion capture and affinity properties. Properties. A preferred class of compounds can be used to bind commercial extractant molecules with... The groups are prepared by chemical bonding. For example, one bifunctional complexing agent used in this embodiment is perfluorononyloxy-di( Ethylhexyl phosphate, its structural formula is (P507 is the second ( (Abbreviation for ethylhexyl phosphate).

[0022] Its synthetic route is as follows: First, perfluorononyl iodine ( Hydrolysis under alkaline conditions yields perfluorononanol ( ), and then it reacted with phosphorus pentoxide ( The reaction produces perfluorononyl phosphate dichloro, which is then reacted with... The target product is obtained by reacting ethylhexanol. This compound in C, 20 MPa Its solubility was determined to be greater than 0.15 mol / L.

[0023] Under the same conditions (60°C, 20 MPa), the tested ion trapping ends were all P507, which is compatible with... The ends are perfluorohexyl ( ), perfluorooctyl ( ), perfluorododecyl ( The properties of complexing agents were observed. It was found that... Chain length complexing agents in The solubility of all samples is >0.1 mol / L, and the selectivity for rare earth elements (RE / Ca separation factor) is significantly higher than that of other rare earth elements. . Although it has better solubility, the synthesis cost is higher, so it is a comprehensive consideration. This is the optimal range.

[0024] Example 1 1. Raw material preparation: Take rare earth calcium slag (mainly composed of) produced by a rare earth smelter Total rare earth oxides (TREO): ~3.5% were used as the treatment target.

[0025] 2. Pretreatment: The aforementioned rare earth calcium slag was fed into an air jet mill for mechanical pulverization. By controlling the airflow pressure and the classifier speed, the material was pulverized to a median particle size D50 of approximately 75 micrometers, with over 95% of the particles having a particle size between 30 and 180 micrometers.

[0026] The pulverized particles were placed in a vacuum drying oven, with a vacuum level of 50 Pa and a temperature of 120°C, and heated continuously for 4 hours. After processing, samples were taken for testing, and the moisture content was found to be less than 0.1 wt%.

[0027] 3. Preparation of extraction medium: Provide the above bifunctional complexing agent .

[0028] liquid The pressure was increased to 20 MPa using a high-pressure pump, and then heated to 60 °C using a heater to reach a supercritical state. The bifunctional complexing agent was then continuously injected into the supercritical fluid at a concentration of 0.1 mol / L. In the fluid, they are thoroughly mixed to form a homogeneous extraction medium.

[0029] 4. Supercritical fluid extraction: The high-pressure extraction system is designed in accordance with ASME standards and is equipped with safety relief devices (rupture discs and safety valves), pressure sensors, and an interlocking emergency shutdown system to ensure safe operation.

[0030] 500 grams of pretreated rare earth calcium slag particles were loaded into a 316L stainless steel stirred high-pressure extraction vessel, which was equipped with a sintered metal sieve plate to support the slag. The extraction medium was pumped into the vessel from the bottom at a flow rate of 0.1 kg / h, and the magnetic stirring system was started to ensure full contact between the solid and liquid phases. The operating temperature was maintained at 60°C and the pressure at 20 MPa.

[0031] 5. Separation and recycling: Supercritical fluids loaded with rare earth complexes Fluid flows out from the top of the extraction vessel and enters the back pressure regulating valve. Here, the fluid pressure drops sharply to 0.5 MPa, and the temperature is cooled to 35 °C via a heat exchanger. Supercritical It rapidly transforms into a gaseous state, its solubility decreases sharply, and the rare earth-complex precipitates out as a light yellow powdery solid, which is then efficiently collected in a cyclone separator.

[0032] Analysis of the collected solid products revealed them to be rare earth complexes, with extremely low levels of non-rare earth impurities (Ca, F) (Ca: 45 ppm, F: 38 ppm).

[0033] 6. Regeneration and Recycling: Complexing agent regeneration: The collected rare earth-complex solid was mixed with an aqueous acetic acid solution at pH 4.0 at a mass ratio of 1:10 and stirred at 50°C for 1 hour. The rare earth ions were dissociated into the aqueous phase, and the regenerated complexing agent was then regenerated. The complexing agent is enriched in the organic phase. After settling and separation, the organic phase is recovered by vacuum distillation in a closed explosion-proof system to obtain a pure bifunctional complexing agent with a single recovery rate of over 98%, which can be recycled back to step 3. The rare earth ions in the aqueous phase are precipitated with oxalic acid and then ignited to obtain rare earth oxides with a purity greater than 99.5%.

[0034] The complexing agent was continuously regenerated and recycled 20 times. At the 20th cycle, its cumulative recovery rate was still higher than 95.5%, the rare earth extraction rate remained at 94.8%, and the total amount of Ca and F impurities in the rare earth product was <700ppm.

[0035] The acidic wastewater generated during the regeneration process is collected and entrusted to a qualified hazardous waste treatment unit for compliant disposal.

[0036] Circulation: Gaseous phase exiting the cyclone separator After being liquefied by a compressor, the liquid sequentially passes through an activated carbon adsorption tower and a 3A molecular sieve drying tower (used molecular sieves that are replaced periodically are disposed of according to hazardous waste management regulations) to remove any trace amounts of complexing agents and moisture that may be entrained. The purified liquid... It is sent to a storage tank and recycled for the next extraction. The entire process... The calculated recovery rate for a single cycle is greater than 98%.

[0037] 7. Harmlessness: The residue after extraction was subjected to a toxicity leaching test (TCLP) according to the "Identification Standard for Hazardous Waste" (GB 5085.3-2007). The test results showed that the leaching concentration of fluoride was 4.5 mg / L, far below the standard limit (100 mg / L). This proves that the residue poses no risk of toxic leaching and can be treated as general solid waste for landfill disposal, achieving the goal of harmlessness.

[0038] Example 2 The extraction temperature was set to 85℃, and the extraction pressure was set to 25 MPa. The concentration of the bifunctional complexing agent was changed to 0.03 mol / L. The drying conditions were changed to drying at 150℃ under normal pressure for 2 hours.

[0039] The remaining steps and conditions are exactly the same as in Example 1.

[0040] Example 3 Modified complexing agent: The ion trapping end is replaced with thiophenecarboxylic acid trifluoroacetone (HTTA, a β-diketone), which is more resistant to ion chelation. The end-chain was replaced with a polydimethylsiloxane chain (PDMS, Mn≈1000). This bifunctional complexing agent was effective at 50°C and 15 MPa. The solubility is approximately 0.08 mol / L. The concentration of this bifunctional complexing agent was changed to 0.06 mol / L.

[0041] The extraction temperature was adjusted to 50℃ and the pressure to 15 MPa.

[0042] The remaining steps and conditions are the same as in Example 1.

[0043] Comparative Example 1: Traditional acid leaching process Take 500 grams of rare earth calcium slag raw material from the same batch as in Example 1, and after undergoing the same pulverizing and drying treatment as in Example 1, place it in a reactor. Add 2.5 L of 3 mol / L hydrochloric acid solution, and stir and leach at 70°C for 4 hours.

[0044] After the reaction, solid-liquid separation is performed to obtain an acidic leachate rich in rare earth elements but also containing large amounts of calcium and fluoride ions, and fluoride-containing waste residue. The leachate requires subsequent multi-stage solvent extraction and precipitation to separate and purify the rare earth elements, and generates a large amount of acidic and fluoride-containing wastewater that needs to be treated.

[0045] Comparative Example 2: Traditional Fire Process Take 500g of rare earth calcium slag raw material from the same batch as in Example 1, and after pulverizing and drying in the same way as in Example 1, mix it with 100g of sodium carbonate, place it in a graphite crucible, heat it in a muffle furnace at 1500℃ for 4 hours, and purge it with nitrogen.

[0046] After cooling, the molten slag is broken up, and the rare earth elements are leached out with 5% hydrochloric acid and filtered to obtain a rare earth solution.

[0047] Comparative Example 3: Lack of Parental Relationship End-of-terminal complexing agents The bifunctional complexing agent (P507-C9F19) used in Example 1 was replaced with an equimolar amount of ordinary P507 (di(2-ethylhexyl) phosphate). Without adding any polar modifiers, an attempt was made to dissolve it in pure supercritical fluid. An extraction medium is formed in (60°C, 20MPa).

[0048] Effect Comparison The test results of Examples 1-3 and Comparative Examples 1-3 were detected and calculated, and the results are summarized in Table 1.

[0049]

[0050] As shown in Table 1, the process of this invention achieves a high rare earth recovery rate, with the recovery rates of Examples 1-3 all exceeding 92% (up to 96.8%). More notably, it exhibits extremely high selectivity in the extraction of rare earth ions, with very low levels of co-extracted calcium and fluorine impurities (Ca < 100 ppm, F < 130 ppm), resulting in a final rare earth product purity exceeding 99.0% (up to 99.5%). This contrasts sharply with Comparative Example 1 (acid leaching method), which produces a leachate with high impurity concentrations (Ca 15 g / L, F 2 g / L), and Comparative Example 2 (pyrometallurgical method), which still requires subsequent acid leaching purification, demonstrating the fundamental advantages of the bifunctional complexing agent.

[0051] In contrast, the ordinary P507 used in Comparative Example 3 was in pure... Its inherent solubility in [the medium] is extremely low (measured <0.003 mol / L), making it impossible to form a homogeneous extraction medium with the required concentration (0.01-0.5 mol / L). Due to insufficient effective concentration, the small amount of dissolved P507 molecules cannot pass through [the medium]. The reaction interface effectively transported to the rare earth calcium slag cannot effectively coordinate with the encapsulated rare earth ions. The final rare earth extraction rate is less than 5%. Even with attempts to enhance mixing through technical means (such as vigorous stirring and ultrasound), it is impossible to form a homogeneous extraction medium of the required concentration (0.01-0.5 mol / L).

[0052] This invention achieves efficient recycling of core materials. Examples 1-3: Supercritical The recycling rates of both solvents and bifunctional complexing agents exceed 98%. This closed-loop design avoids the problems of large amounts of acidic and fluoride-containing wastewater generated by traditional wet processes (Comparative Example 1) and high energy consumption and potential exhaust gas pollution of pyrometallurgical processes (Comparative Example 2) in principle, thereby achieving the green goal of resource-based waste treatment and near-zero pollution emissions.

[0053] Examples 1-3 all achieved efficient extraction under medium-low temperatures (50-85℃) and medium pressures (15-25 MPa), eliminating the need for the extreme high temperature of 1500℃ in Comparative Example 2. These mild conditions directly resulted in a significant reduction in energy consumption, while also reducing the requirements for the equipment's high-temperature resistance and corrosion resistance, thus reducing investment and operating costs.

[0054] This invention utilizes the unique properties of supercritical fluids to achieve product-solvent separation with only simple physical decompression, resulting in a highly efficient and rapid process. In contrast, the acid leaching method in Comparative Example 1 requires complex subsequent multi-stage extraction and wastewater treatment steps, while the pyrometallurgical method in Comparative Example 2 also necessitates crushing and re-leaching. The simplified process of this invention directly leads to improved operational efficiency and reduced overall costs.

[0055] In summary, this invention provides an innovative process that can simultaneously solve the problems of resource recycling, environmental protection, and economic benefits. Compared with existing technologies, it has achieved breakthrough progress in terms of recovery rate, selectivity, environmental friendliness, and economy, providing a brand-new solution for the harmless and resource-based treatment of rare earth calcium slag.

Claims

1. A green and harmless treatment process for rare earth calcium slag, characterized in that, The process includes the following steps: (a) Pretreatment: The rare earth calcium slag is mechanically crushed to obtain particles with an average particle size of 20 micrometers to 200 micrometers, and the particles are dried to remove moisture. (b) Providing a bifunctional complexing agent: the bifunctional complexing agent comprises an ion-trapping terminal for selectively chelating rare earth ions and a hydrophilic terminal for improving their solubility in supercritical carbon dioxide. end; (c) Forming an extraction medium: The bifunctional complexing agent is dissolved in supercritical carbon dioxide fluid to form an extraction medium, wherein the concentration of the bifunctional complexing agent is in the range of 0.01 mol / L to 0.2 mol / L; (d) Extraction contact: Under extraction conditions of 40°C to 100°C and 10 MPa to 30 MPa, the extraction medium is brought into contact with the pretreated rare earth calcium slag particles, so that the bifunctional complexing agent reacts with the rare earth elements to generate rare earth-complexes and dissolves in supercritical carbon dioxide fluid. (e) Separation and recovery: The supercritical carbon dioxide fluid loaded with the rare earth-complex is subjected to depressurization to reduce its pressure to below the critical pressure, thereby causing the rare earth-complex to precipitate and be separated and collected; The ion-trapping terminal in step (b) comprises at least one functional group selected from phosphate ester, phosphonic acid, carboxylic acid, β-diketone, hydroxyl, and amino groups; the hydrophilic... Perfluoroalkyl chains with 8-12 carbon atoms at the end; Step (e) is followed by step (f) complexing agent regeneration: the separated and collected rare earth-complex is dissociated to release rare earth ions and regenerate the bifunctional complexing agent. The regenerated bifunctional complexing agent is returned to step (c) for recycling. The dissociation process is achieved by washing with a weakly acidic aqueous solution with a pH of 3-5; Step (e) is followed by step (g) carbon dioxide recycling: the gaseous carbon dioxide after the rare earth-complex is separated is compressed and purified and reused to form the extraction medium described in step (c).

2. The process according to claim 1, characterized in that, The drying conditions described in step (a) are: heating at 100 to 150 degrees Celsius for 2 to 6 hours, or drying under vacuum conditions.

3. The process according to claim 2, characterized in that, The ion-trapping terminal is a functional group containing a phosphate ester group or a phosphonic acid group; the hydrophilic... The terminal is a perfluoroalkyl chain with 6-20 carbon atoms.

4. The process according to claim 1, characterized in that, The extraction conditions described in step (d) are: temperature 60 degrees Celsius and pressure 20 MPa.

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