Process for recycling precious metals from waste circuit boards

Abstract

The application provides a process for recycling rare and precious metals from waste circuit boards, and belongs to the field of waste resource recycling. The process comprises: microwave pyrolysis pretreatment of waste circuit boards to obtain pretreated materials; leaching of the pretreated materials with a strong alkali solution to obtain a tin-containing filtrate and first filter residue; mixing of the first filter residue, an acid leaching solution containing Fe 3+ , and a magnetic nanometer composite catalysis-complexing agent, followed by leaching under the assistance of an ultrasonic field and electrochemical oxidation to obtain a leaching solution; recovery of the magnetic nanometer composite catalysis-complexing agent to obtain a precious solution containing copper, nickel, zinc, and aluminum and second filter residue; leaching of the second filter residue mixed with a gold leaching system solution to obtain a precious solution containing gold and silver; and recovery of corresponding metals from the tin-containing filtrate, the precious solution containing copper, nickel, zinc, and aluminum, and the precious solution containing gold and silver, respectively. The application realizes efficient, high-selectivity, and synergistic recovery of various rare and precious metals such as tin, copper, nickel, zinc, aluminum, gold, and silver in circuit boards.

Description

Technical Field

[0001] This application relates to the field of waste resource recycling technology, and in particular to a process for recycling rare and precious metals from waste circuit boards. Background Technology

[0002] Waste printed circuit boards (PCBs) have a complex composition and are an important secondary resource containing various rare and precious metals. Current mainstream recycling technologies have significant limitations, making it difficult to achieve efficient and selective synergistic recovery of multiple metals. Pyrometallurgical processes (such as incineration and smelting) can process large quantities of material, but they are extremely energy-intensive and prone to producing highly toxic gases such as dioxins at high temperatures. They also cause metals to melt together to form alloys or enter the slag, resulting in low metal recovery rates, difficulties in separation and purification, and low product value. Physical sorting methods (such as crushing, magnetic separation, and electrostatic separation) heavily rely on the degree of dissociation between metals and non-metallic components, have poor recovery effects on fine-grained metals, and cannot achieve complete separation between metals with different chemical forms.

[0003] Hydrometallurgy offers greater selectivity, but traditional processes suffer from systemic drawbacks. Conventional multi-step acid leaching processes are lengthy, often requiring leaching of different metals at varying acidities and potentials. Inter-step washing and neutralization operations are cumbersome, resulting in high acid and alkali consumption and susceptibility to displacement reactions between metal ions (such as Sn). 2+ The substitution of Cu (Cu) causes cross-contamination and metal loss. While cyanidation or aqua regia methods are effective for precious metal recovery, they use highly toxic or corrosive reagents, posing significant environmental risks. Furthermore, catalysts or complexing agents added to increase leaching rates are mostly homogeneous systems, difficult to separate and recover from the reaction system, leading to increased costs and new wastewater treatment challenges. Therefore, developing a short-process, highly efficient, highly selective, and environmentally friendly synergistic recovery process for multiple rare and precious metals has become an urgent technological need in this field. Summary of the Invention

[0004] This application provides a process for recycling rare and precious metals from waste circuit boards to solve the following technical problem: how to achieve efficient, selective, and synergistic recycling of various rare and precious metals such as tin, copper, nickel, zinc, aluminum, gold, and silver from circuit boards.

[0005] This application provides a process for recycling rare and precious metals from waste circuit boards, the process including the following steps: S1. Disassemble and crush the waste circuit boards, and perform microwave pyrolysis pretreatment to obtain pretreated materials; S2. The pretreated material is subjected to a first leaching with a strong alkaline solution, and after solid-liquid separation, a tin-containing filtrate and a first filter residue are obtained. S3. The first filter residue containing Fe 3+An acidic leachate and a magnetic nanocomposite catalyst-complexing agent are mixed, and then a second leaching is performed under the assistance of an ultrasonic field and electrochemical oxidation to obtain a leachate. The magnetic nanocomposite catalyst-complexing agent includes: a magnetic core, a nitrogen-doped carbon layer coated on the surface of the magnetic core, and an active component loaded on the nitrogen-doped carbon layer. The active component includes: phosphotungstic acid, iminodiacetic acid, and a quaternary ammonium salt cationic surfactant. S4. The magnetic nanocomposite catalyst-complexing agent is recovered by magnetic separation, followed by solid-liquid separation to obtain a precious liquid containing copper, nickel, zinc and aluminum and a second filter residue. S5. The second filter residue is mixed with the gold leaching system solution for a third leaching. After solid-liquid separation, a precious solution containing gold and silver is obtained. The gold leaching system solution includes: sodium thiosulfate, copper sulfate, ammonia, ammonium sulfite and trisodium citrate. S6. Recover the corresponding metals from the tin-containing filtrate, the precious solution containing copper, nickel, zinc, and aluminum, and the precious solution containing gold and silver, respectively.

[0006] Optionally, the particle size of the pretreated material is 0.3–0.8 mm; The microwave pyrolysis pretreatment is carried out under an inert atmosphere at a temperature of 450–500°C, a holding time of 15–30 min, and a microwave power of 600–1500 W / kg.

[0007] Optionally, the strong alkaline solution is a sodium hydroxide solution with a concentration of 2-4 mol / L; The first leaching includes the following parameters: temperature of 80-95℃, time of 2-3h, and liquid-solid ratio of (4-6):1.

[0008] Optionally, the Fe-containing 3+ In the acidic leachate, Fe 3+ The concentration is 25–30 g / L; The first filter residue and the Fe-containing 3+ The liquid-to-solid ratio of the acidic leachate mixture is (4-8):1; The second leaching process includes the following parameters: initial pH value of 1.5 to 2.0, temperature of 50 to 60°C, and time of 2 to 4 hours; The frequency of the ultrasonic field is 20–50 kHz, and the power density is 50–150 W / L.

[0009] Optionally, the added mass of the magnetic nanocomposite catalyst-complexing agent is equal to the mass of the Fe-containing... 3+ The acidic leachate accounts for 0.5 to 1.5% of the total mass of the first filter residue.

[0010] Optionally, the preparation method of the magnetic nanocomposite catalyst-complexing agent includes the following steps: Fe3O4 nanoparticles were obtained by dissolving ferric salts and ferrous salts in water and carrying out a hydrothermal reaction at 160–200 °C for 6–12 h under an inert atmosphere. The Fe3O4 nanoparticles were dispersed in an aqueous solution of citric acid and urea, and then subjected to a hydrothermal reaction at 180–200 °C for 8–16 h to form a nitrogen-doped carbon shell rich in carboxyl and amino groups on the surface of the Fe3O4 nanoparticles, thus obtaining the Fe3O4@C-COOH / NH2 composite material. The Fe3O4@C-COOH / NH2 composite material was dispersed in an aqueous solution of phosphotungstic acid and stirred at 30–50 °C for 4–8 h to immobilize the phosphotungstic acid on the surface of the carbon layer, thus obtaining an intermediate supported by phosphotungstic acid. The intermediate was dispersed in a buffer solution, and iminodiacetic acid was added. In the presence of condensing agents 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide, the mixture was stirred at pH 4.5–5.5 and 40–60°C for 12–24 h to allow the amino group of iminodiacetic acid to covalently graft onto the carboxyl group on the surface of the carbon layer through an amidation reaction, thereby obtaining a complexed functionalized intermediate. The complexed functionalized intermediate was dispersed in ethanol, and a quaternary ammonium salt cationic surfactant was added. After stirring and impregnation at room temperature for 2-6 hours, the solvent was removed so that the quaternary ammonium salt cationic surfactant modified the surface of the complexed functionalized intermediate, thereby obtaining the magnetic nanocomposite catalyst-complexing agent.

[0011] Optionally, the molar ratio of the trivalent ferric salt to the divalent ferric salt is (1.5–2.5):1; The mass ratio of the Fe3O4 nanoparticles, the citric acid and the urea is 1:(2-5):(1-3); The concentration of the phosphotungstic acid aqueous solution is 10–50 g / L, and the mass ratio of the Fe3O4@C-COOH / NH2 composite material to phosphotungstic acid is 1:(0.1–0.5). The mass ratio of the intermediate to iminodiacetic acid is 1:(0.2-0.8), and the molar ratio of 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride to iminodiacetic acid is (1-1.5):1; The quaternary ammonium salt cationic surfactant is hexadecyltrimethylammonium bromide, and the mass of hexadecyltrimethylammonium bromide is 5-20% of the mass of the complexed functionalized intermediate.

[0012] Optionally, in the magnetic nanocomposite catalyst-complexing agent, the Fe3O4 nanoparticles have a particle size of 30-80 nm, the phosphotungstic acid loading is 5-15 wt%, and the iminodiacetic acid grafting amount is 3-8 wt%.

[0013] Optionally, in the gold immersion system, the concentrations of each component are: sodium thiosulfate 0.15–0.30 mol / L, copper sulfate 0.02–0.06 mol / L, ammonia 0.8–2.0 mol / L, ammonium sulfite 0.05–0.15 mol / L, and trisodium citrate 0.02–0.10 mol / L.

[0014] Optionally, the third leaching includes the following parameters: liquid-to-solid ratio of (5-8):1, pH value of 9.5-10.5, temperature of 55-65℃, and leaching time of 4-8h.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a process for recycling rare and precious metals from waste circuit boards. It solves the selectivity problem through a three-step sorting process and solves the kinetics and passivation problems of main metal leaching through magnetic nano-catalysis-complexing agent. It systematically achieves efficient, highly selective and synergistic recycling of multiple rare and precious metals in circuit boards.

[0016] First, the process achieves highly selective, step-by-step leaching of metals through precise step sequencing, preventing mutual inhibition between metals from the outset. The first step, alkaline tin leaching, is specifically designed to dissolve the amphoteric metal tin. In concentrated alkaline solution, tin is efficiently converted into sodium stannate and enters the solution, while copper, nickel, gold, and silver remain stable and accumulate in the filter residue under these conditions. This front-end diversion strategy completely solves the interference of tin ions on subsequent copper leaching in traditional acidic processes. It avoids metal cross-contamination, encapsulation, and loss caused by tin replacing copper to form sponge copper, creating a pure material basis for the high-purity leaching of the main metal. After the main metal recovery, the precious metal slag undergoes a third leaching through a targeted thiosulfate system. This system exhibits excellent selective complexation and dissolution capabilities for gold and silver under alkaline conditions, while residual small amounts of base metals and inert components remain insoluble, thus achieving green and efficient separation of precious metals. This process design, based on the progressive separation of metal chemical properties, forms the macroscopic framework for achieving high selectivity.

[0017] Secondly, addressing the slowest and most challenging leaching kinetics in the main metal recovery stage, this application introduces a self-developed magnetic nanocomposite catalyst-complexing agent. This agent constructs a synergistic mechanism of oxidation and complexation at the nanoscale, achieving highly efficient recovery. This material is not a simple mixture, but a meticulously designed multifunctional integrated platform. Its core mechanism is the nanoscale synergy of interfacial oxidation and adjacent complexation. Phosphotungstic acid, immobilized on a nitrogen-doped carbon layer, acts as a strong oxidant, directly abstracting electrons from the metal particle surface and efficiently oxidizing elemental metals. Its heterogeneous catalytic pathway is far superior to that dependent on Fe. 3+Traditional oxidation processes involving ion diffusion are rapid. The key lies in the iminodiacetic acid complex site, spatially adjacent to the phosphotungstic acid site, which can promptly capture freshly dissolved metal ions. This serves three purposes: first, it removes the product from the reaction interface, continuously driving the dissolution equilibrium forward; second, it prevents metal ions from reprecipitating or hydrolyzing on the particle surface to form a passivation film; and third, it utilizes the iminodiacetic acid's affinity for Cu... 2+ Ni 2+ The high complexation constant enables selective enrichment of target ions. Simultaneously, the magnetic iron oxide core and protective carbon layer allow for instantaneous recovery and recycling of the catalyst after reaction via a magnetic field, solving the global challenge of recovering homogeneous catalysts. This is further enhanced by ultrasonic field-enhanced mass transfer and electrochemical assistance to achieve Fe... 3+ / Fe 2+ This combined mechanism of recycling improves the leaching kinetics and final recovery rate of main metals such as copper and nickel to near-complete levels.

[0018] Finally, the entire process embodies a high degree of synergy and a closed-loop cycle, jointly ensuring the overall high efficiency and environmental friendliness of the recycling process. Fe in the second leaching... 3+ The oxidant is regenerated in situ within the system via electrochemical means, and the catalyst is circulated through magnetic separation, forming an internal cycle for the core reagents. Ammonium sulfite in the gold leaching system acts as a stabilizer, effectively inhibiting the decomposition of sodium thiosulfate and maintaining the long-term stability of the leaching system. The solutions produced in each step are routed to dedicated metal recovery units, ultimately converting the ions into high-purity products. This process, with its internal circulation of key substances and differentiated productization pathways, optimizes metal recovery efficiency while minimizing material consumption and environmental impact, achieving a synergistic balance between high efficiency, high selectivity, and environmental benefits. Attached Figure Description

[0019] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 A schematic flowchart illustrating a process for recycling rare and precious metals from waste circuit boards, provided as an embodiment of this application; Figure 2 This is a low-magnification TEM image of the magnetic nanocomposite catalyst-complexing agent provided in Example 1 of this application; Figure 3This is a high-magnification TEM image of the magnetic nanocomposite catalyst-complexing agent provided in Example 1 of this application; Figure 4 The infrared spectrum of the magnetic nanocomposite catalyst-complexing agent provided in Example 1 of this application. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] Figure 1 This is a schematic flowchart illustrating a process for recycling rare and precious metals from waste circuit boards, provided as an embodiment of this application.

[0024] like Figure 1 As shown in the figure, this application provides a process for recycling rare and precious metals from waste circuit boards, which includes the following steps: S1. Disassemble and crush the waste circuit boards, and perform microwave pyrolysis pretreatment to obtain pretreated materials; S2. The pretreated material is subjected to a first leaching with a strong alkaline solution, and after solid-liquid separation, a tin-containing filtrate and a first filter residue are obtained. S3, the first filter residue containing Fe 3+ An acidic leachate and a magnetic nanocomposite catalyst-complexing agent are mixed, and then a second leaching is carried out under the assistance of ultrasonic field and electrochemical oxidation to obtain a leachate. The magnetic nanocomposite catalyst-complexing agent includes: a magnetic core, a nitrogen-doped carbon layer coated on the surface of the magnetic core, and active components loaded on the nitrogen-doped carbon layer. The active components include: phosphotungstic acid, iminodiacetic acid, and quaternary ammonium salt cationic surfactant. S4. The magnetic nanocomposite catalyst-complexing agent is recovered by magnetic separation, followed by solid-liquid separation to obtain a precious liquid containing copper, nickel, zinc and aluminum and a second filter residue. S5. The second filter residue is mixed with the gold leaching system solution for a third leaching. After solid-liquid separation, a precious solution containing gold and silver is obtained. The gold leaching system solution includes: sodium thiosulfate, copper sulfate, ammonia, ammonium sulfite and trisodium citrate. S6. Recover the corresponding metals from the tin-containing filtrate, the precious solution containing copper, nickel, zinc, and aluminum, and the precious solution containing gold and silver, respectively.

[0025] It should be noted that the dismantling, crushing, and microwave pyrolysis pretreatment in step S1 mainly completes the physical desorption and purification of raw materials. By exposing the metal surface through mechanical crushing, and then using microwave pyrolysis under an inert atmosphere, the organic resin is pyrolyzed and volatilized, thereby eliminating the interference of organic matter in the subsequent wet leaching, and possibly activating the metal through thermal stress, to obtain a pretreated material mainly composed of metals and inorganic substances.

[0026] Step S2, the first leaching with a strong alkaline solution, utilizes the amphoteric nature of tin to preferentially and selectively leach tin, generating a sodium stannate solution. This step separates tin at the beginning of the process, preventing tin from interfering with the leaching of the main metal (such as copper) in the subsequent acidic system, thus resolving the issue of conflicting metal recovery sequences. The tin-containing filtrate is used to recover tin products, while the filter residue becomes an intermediate product enriched with copper, nickel, zinc, aluminum, and other precious metals.

[0027] Step S3 is the core step in recovering main metals such as copper, nickel, zinc, and aluminum. Its disruptive efficiency stems from the innovative magnetic nanocomposite catalyst-complexing agent coupled with ultrasonic and electrochemical multi-field processes. This catalyst is a precisely designed multifunctional nanoreaction platform. Its magnetic iron oxide core provides superparamagnetism, ensuring rapid separation (from seconds to tens of seconds) after the reaction via an external magnetic field, with a separation efficiency >95%, solving the major engineering challenge of recovering homogeneous catalysts. The nitrogen-doped carbon layer coating the magnetic core provides a physical barrier within the set leaching cycle, acting as an inert barrier to protect the core from corrosion by acidic media. On the other hand, its good conductivity promotes interfacial electron transport, and its abundant amino functional groups on the surface serve as chemical anchors for immobilizing the active components.

[0028] The surface bifunctional modification of the catalyst is key to its mechanism of action. Phosphotungstic acid immobilized on the carbon layer acts as a multi-electron transfer medium, and its redox potential can assist Fe... 3+ The system accelerates metal oxidation kinetics compared to Fe, which depends on bulk diffusion. 3+ The presence of ions in this interfacial catalytic oxidation significantly accelerates the reaction kinetics. The iminodiacetic acid group covalently grafted onto the catalyst surface via amidation provides a strong bidentate chelating site for Cu. 2+ Ni 2+Plasma possesses a high complexation constant. The most crucial design element lies in the close spatial adjacency of the oxidation and complexation centers at the nanoscale. This configuration achieves synergy between oxidation and complexation: phosphotungstic acid acts as an electron transfer medium to promote metal oxidation and dissolution, while iminodiacetic acid stabilizes metal ions in solution through complexation. Their complementary functions synergistically drive the leaching reaction forward. This allows for the removal of products from the reaction interface, continuously promoting a positive shift in the dissolution equilibrium; secondly, it prevents metal ions from reprecipitating or undergoing hydrolytic passivation on the solid surface; and thirdly, it enables in-situ selective enrichment of target metal ions. The added quaternary ammonium salt-type cationic surfactant is used to improve the dispersion stability of nanoparticles in complex slurry systems. Under the action of this catalyst, Fe in the solution... 3+ Primarily acting as an electron transfer intermediary, the Fe produced by reduction 2+ Regenerated at the anode to Fe under electrochemical oxidation assistance 3+ This design enables the leaching agent to circulate within the field. Simultaneously applied ultrasonic fields utilize cavitation effects to strongly disturb the interface and renew the reaction surface, further enhancing mass transfer. This series of designs allows for efficient completion of main metal leaching within a few hours.

[0029] Step S4 recovers the catalyst through magnetic separation, and then obtains a precious liquid containing the main metal ions and a second filter residue rich in precious metals through solid-liquid separation, which plays a key role in material diversion and core material recycling.

[0030] Step S5 is specifically designed for the green extraction of gold and silver, and its core is the environmentally friendly thiosulfate leaching system. In this system, sodium thiosulfate is the main ligand, forming stable complex anions with gold and silver under alkaline conditions, thus dissolving them. Copper sulfate and ammonia form an oxidation-catalysis system: ammonia maintains the alkaline environment and prevents the decomposition of sodium thiosulfate, while simultaneously reacting with Cu... 2+ The process forms a copper-ammonia complex ion, which acts as an oxidant and catalytic intermediate, promoting the oxidative dissolution of gold and silver. The addition of ammonium sulfite is crucial, effectively inhibiting the disproportionation and decomposition of sodium thiosulfate and the formation of harmful byproducts such as polythionate, significantly improving the chemical stability and recyclability of the leaching system. Trisodium citrate acts as a buffer and auxiliary complexing agent, helping to stabilize the system pH and complex potentially interfering ions. This system achieves highly selective leaching of gold and silver under mild conditions, completely avoiding the use of cyanide or aqua regia.

[0031] Step S6 is the final productization stage, in which high-purity metals such as tin, copper, nickel, zinc, aluminum, gold, and silver or their high-value-added compounds are recovered from each section of precious liquid through mature unit operations such as electrolysis, precipitation, and crystallization.

[0032] In some embodiments, the particle size of the pretreated material is 0.3–0.8 mm; The microwave pyrolysis pretreatment was carried out under an inert atmosphere at a temperature of 450–500℃, a holding time of 15–30 min, and a microwave power of 600–1500 W / kg.

[0033] Crushing materials to a particle size of 0.3–0.8 mm can ensure the full dissociation of metals and non-metals while avoiding difficulties in subsequent solid-liquid separation caused by excessively fine particles.

[0034] Microwave pyrolysis is carried out under an inert atmosphere at a temperature range of 450–500°C. This temperature ensures that organic polymers such as epoxy resin are fully decomposed into small-molecule volatiles, thereby completely eliminating interference from organic matter in subsequent leaching and exposing the encapsulated metal. A holding time of 15–30 minutes is sufficient to complete the pyrolysis process while avoiding unnecessary energy consumption. A microwave power density of 600–1500 W / kg provides rapid and uniform bulk heating for the reaction, and its selective heating characteristics help to generate microcracks within the metal particles, activating the metal surface.

[0035] In some embodiments, the strong alkali solution is a sodium hydroxide solution with a concentration of 2-4 mol / L; The first leaching process includes the following parameters: temperature of 80–95℃, time of 2–3 h, and liquid-to-solid ratio of (4–6):1.

[0036] A sodium hydroxide solution with a concentration of 2–4 mol / L can provide sufficiently high alkalinity to efficiently dissolve tin and its oxides, forming sodium stannate.

[0037] Maintaining the leaching temperature within the high-temperature range of 80–95°C significantly accelerates the kinetics of tin dissolution. A leaching time of 2–3 hours ensures near-complete tin leaching. Controlling the liquid-to-solid ratio to (4–6):1 provides sufficient mass transfer interface and solution capacity for the solid-liquid reaction while maintaining the economy of subsequent processing. This step separates tin at the source, clearing the way for subsequent acidic leaching of the main metal.

[0038] In some embodiments, it contains Fe 3+ In the acidic leachate, Fe 3+ The concentration is 25–30 g / L; The first filter residue contains Fe 3+ The liquid-to-solid ratio of the acidic leachate mixture is (4-8):1; The second leaching process includes the following parameters: initial pH value of 1.5 to 2.0, temperature of 50 to 60°C, and time of 2 to 4 hours; The frequency of the ultrasonic field is 20–50 kHz, and the power density is 50–150 W / L.

[0039] Using Fe 3+An acidic leachate with a concentration of 25–30 g / L provides sufficient initial oxidant and a suitable acidic environment (initial pH 1.5–2.0), effectively dissolving some metal oxides while inhibiting premature hydrolysis of certain metal ions. A liquid-to-solid ratio of (4–8):1 provides ample space for solid-phase reaction and catalyst dispersion. A leaching temperature of 50–60 °C strikes a balance between promoting the reaction rate and reducing energy consumption. A leaching time of 2–4 h, enhanced by the catalyst, allows for efficient leaching of the main metal in a relatively short time.

[0040] An ultrasonic field with a frequency of 20–50 kHz and a power density of 50–150 W / L is applied. Its cavitation effect strongly disturbs the solid-liquid interface, continuously renews the reaction surface and prevents particle aggregation, thereby greatly enhancing the mass transfer process.

[0041] In some embodiments, the added mass of the magnetic nanocomposite catalyst-complexing agent is Fe. 3+ The acidic leachate accounts for 0.5 to 1.5% of the total mass of the first filter residue.

[0042] In some embodiments, the preparation method of the magnetic nanocomposite catalyst-complexing agent includes the following steps: Fe3O4 nanoparticles were obtained by dissolving ferric salts and ferrous salts in water and carrying out a hydrothermal reaction at 160–200 °C for 6–12 h under an inert atmosphere. Fe3O4 nanoparticles were dispersed in an aqueous solution of citric acid and urea, and then subjected to a hydrothermal reaction at 180–200 °C for 8–16 h to form a nitrogen-doped carbon shell rich in carboxyl and amino groups on the surface of the Fe3O4 nanoparticles, thus obtaining the Fe3O4@C-COOH / NH2 composite material. The Fe3O4@C-COOH / NH2 composite material was dispersed in an aqueous solution of phosphotungstic acid and stirred at 30–50 °C for 4–8 h to immobilize the phosphotungstic acid on the surface of the carbon layer, thus obtaining an intermediate supported by phosphotungstic acid. The intermediate was dispersed in a buffer solution, and iminodiacetic acid was added. In the presence of condensing agents 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide, the mixture was stirred at pH 4.5–5.5 and 40–60 °C for 12–24 h to allow the amino group of iminodiacetic acid to covalently graft onto the carboxyl group on the surface of the carbon layer through an amidation reaction, thereby obtaining a complexed functionalized intermediate. The complexation functionalized intermediate was dispersed in ethanol, and a quaternary ammonium salt cationic surfactant was added. After stirring and impregnation at room temperature for 2-6 hours, the solvent was removed so that the quaternary ammonium salt cationic surfactant modified the surface of the complexation functionalized intermediate, thus obtaining a magnetic nanocomposite catalyst-complexing agent.

[0043] In some embodiments, the molar ratio of ferric salt to ferrous salt is (1.5–2.5):1; The mass ratio of Fe3O4 nanoparticles, citric acid and urea is 1:(2-5):(1-3); The concentration of the phosphotungstic acid aqueous solution is 10–50 g / L, and the mass ratio of Fe3O4@C-COOH / NH2 composite material to phosphotungstic acid is 1:(0.1–0.5). The mass ratio of the intermediate to iminodiacetic acid is 1:(0.2-0.8), and the molar ratio of 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride to iminodiacetic acid is (1-1.5):1; The quaternary ammonium salt type cationic surfactant is hexadecyltrimethylammonium bromide, and the mass of hexadecyltrimethylammonium bromide is 5 to 20% of the mass of the complexed functionalized intermediate.

[0044] In some embodiments, in the magnetic nanocomposite catalyst-complexing agent, the Fe3O4 nanoparticles have a particle size of 30-80 nm, the loading of phosphotungstic acid is 5-15 wt%, and the grafting amount of iminodiacetic acid is 3-8 wt%.

[0045] Magnetic core formation: Fe3O4 nanoparticles were synthesized by hydrothermal method. The molar ratio of trivalent to divalent iron salts was controlled at (1.5-2.5):1. The reaction was carried out at 160-200℃ for 6-12 hours to obtain superparamagnetic cores with a particle size of 30-80 nm and good magnetic responsiveness.

[0046] Functional carbon shell construction: Using citric acid as the carbon source and urea as the nitrogen source, a nitrogen-doped carbon layer (Fe3O4@C-COOH / NH2) was coated onto the surface of Fe3O4 via a hydrothermal reaction at 180–200℃. The mass ratio of citric acid to urea determined the thickness of the carbon shell and the density of carboxyl and amino functional groups on its surface; these groups serve as anchors for subsequent functionalization.

[0047] Catalytic site immobilization: The above composite material was stirred and adsorbed in a 10–50 g / L phosphotungstic acid solution at 30–50 °C for 4–8 h. Phosphotungstic acid was firmly immobilized on the positively charged nitrogen-doped carbon layer surface through electrostatic and coordination interactions, with the loading controlled at 5–15 wt%. As an excellent multi-electron transfer medium (catalyst), phosphotungstic acid, when immobilized at the interface, can efficiently promote electron transfer from elemental metals (such as Cu). 0 ) Add Fe to the solution 3+ The transfer of Fe greatly accelerated 3+The oxidation kinetics process was investigated. This scheme synthesized a nitrogen-doped carbon layer with both positive and negative charge sites (zwitterionic characteristics) on the surface of magnetic nanoparticles through precursor synthesis. Then, by utilizing the negative charge of PTA in solution and by controlling the environment, PTA was directed to form a strong electrostatic bond with the positive charge centers on the carbon layer, supplemented by possible coordination effects, ultimately achieving stable anchoring of PTA on the surface of the nanoreactor.

[0048] Complexation site grafting: In a buffer system with pH 4.5–5.5, using the condensing agent EDC / NHS, the amino group of iminodiacetic acid is amidated with the carboxyl group on the carbon shell surface at 40–60°C for 12–24 h. The mass ratio of intermediate to IDA is 1:(0.2–0.8), ensuring a grafting amount of 3–8 wt%. Iminodiacetic acid is an effective bidentate chelating ligand, capable of reacting with dissolved Cu in the acidic leaching environment of this process. 2+ Ni 2+ To achieve timely and effective complexation capture of target metal ions.

[0049] Surface property modification: The surface was modified with 5-20% by mass of hexadecyltrimethylammonium bromide, which significantly improved the dispersion stability of nanoparticles in complex slurries and prevented their agglomeration and deactivation.

[0050] During the leaching process, the catalyst operates through a nanoscale synergistic process of "interfacial oxidation-proximity complexation." The immobilized phosphotungstic acid oxidizes and dissolves metal ions (M...) on the metal surface. 0 →M 2+ +2e - After this, the spatially adjacent IDA sites immediately chelate and fix it. This process serves three purposes: first, it disrupts the dissolution equilibrium, propelling the reaction forward; second, it prevents metal ions from redepositing or undergoing hydrolytic passivation; and third, it selectively enriches the target ion. Fe 3+ In the system, it mainly acts as an electron transfer intermediary, and the reduced Fe... 2+ It can be regenerated in real time with electrochemical assistance. Adding the catalyst at 0.5–1.5% of the total system mass provides sufficient active sites. After the reaction, due to its superparamagnetism (saturation magnetization ≥30 emu / g), it can be almost completely recovered within seconds using an external magnetic field, enabling recycling.

[0051] In some embodiments, the concentrations of each component in the gold immersion system are as follows: sodium thiosulfate 0.15–0.30 mol / L, copper sulfate 0.02–0.06 mol / L, ammonia 0.8–2.0 mol / L, ammonium sulfite 0.05–0.15 mol / L, and trisodium citrate 0.02–0.10 mol / L.

[0052] In some embodiments, the third leaching includes the following parameters: liquid-to-solid ratio of (5-8):1, pH value of 9.5-10.5, temperature of 55-65°C, and leaching time of 4-8 hours.

[0053] The synergistic effect of the concentration ranges of the components in the gold leaching system is as follows: 0.15–0.30 mol / L sodium thiosulfate acts as the main ligand, forming stable complex anions with gold and silver; 0.02–0.06 mol / L copper sulfate and 0.8–2.0 mol / L ammonia form an oxidation-catalytic system, with the copper-ammonia complex acting as a catalytic oxidant to promote dissolution; 0.05–0.15 mol / L ammonium sulfite is a key stabilizer, effectively inhibiting the disproportionation decomposition of thiosulfate and the formation of harmful byproducts; 0.02–0.10 mol / L trisodium citrate helps buffer the pH and complex interfering ions. The leaching operation is carried out in an alkaline range of pH 9.5–10.5 and at a temperature of 55–65°C. A leaching time of 4–8 hours combined with a liquid-to-solid ratio of (5–8):1 ensures efficient and selective dissolution of gold and silver.

[0054] This solution achieves a comprehensive improvement in the recycling rate of various metals in waste circuit boards through systematic process restructuring, core material innovation, and multi-field coupling process enhancement, working synergistically from multiple dimensions.

[0055] Firstly, by optimizing the process sequence and implementing front-end diversion, the mutual interference between metals is eliminated, laying the foundation for high recovery rates. Traditional acid processes often suffer from cross-contamination or displacement losses due to the complex metal leaching sequence. This solution innovatively adopts "alkaline preferential tin leaching" as the first step, utilizing the specific solubility of tin in concentrated alkali to efficiently separate it as sodium stannate at the beginning of the process (leaching rate can reach over 95%). This completely avoids the precipitation of tin as a sponge-like metal during the subsequent acidic copper leaching process. This displacement not only encapsulates unreacted copper particles, hindering their further dissolution, but also forms a mixed tin-copper mixture, leading to a decrease in both recovery rates and product purity. Therefore, the front-end diversion strategy fundamentally solves the problem of tin inhibiting the recovery of the main metal, creating a pure material environment for the efficient leaching of metals such as copper and nickel.

[0056] Secondly, for the core metal leaching process, the designed magnetic nanocomposite catalyst-complexing agent breaks through the kinetic limitations and passivation problems of traditional leaching in principle through the nanoscale synergistic mechanism of "interfacial oxidation-nearby complexation", thereby pushing the leaching rate of metals such as copper and nickel to near complete (>98%).

[0057] Enhanced oxidation kinetics: Phosphotungstic acid immobilized on the catalyst surface acts as a multi-electron oxidant, exhibiting a higher redox potential than conventionally used Fe. 3+ / Fe 2+Electron couples can be used, and electron transfer can occur directly on the surface of solid metal particles (heterogeneous catalysis), resulting in a shorter reaction pathway and far greater efficiency than Fe-dependent reactions. 3+ Homogeneous oxidation process involving diffusion of ionic phases.

[0058] Disruption of dissolution equilibrium and prevention of passivation: The iminodiacetic acid complexation sites on the catalyst surface, spatially adjacent to the phosphotungstic acid sites, can instantly capture dissolved metal ions (such as Cu). 2+ This "dissolution-capture" mechanism removes the product from the reaction interface, continuously pushing the dissolution equilibrium toward the positive reaction. On the other hand, it effectively avoids the reprecipitation or hydrolysis of metal ions on the particle surface due to local oversaturation, thus maintaining the reaction surface in a highly active state.

[0059] Improved mass transfer and selective enrichment: The applied ultrasonic field strongly perturbs the solid-liquid interface and renews the reaction surface through cavitation, greatly enhancing the mass transfer process between reactants and products. Simultaneously, iminodiacetic acid affects Cu... 2+ Ni 2+ Its high selective complexing ability enables it to be used in Fe-rich environments. 3+ The target metal is preferentially enriched in the complex solution of plasma, which improves the selectivity and efficiency of the leaching process.

[0060] Furthermore, by constructing a closed-loop circulation system, the core reaction conditions were stabilized, ensuring the sustainability of the high recovery rate. In the second leaching step, the consumed oxidant Fe... 3+ Real-time regeneration of (Fe) using electrochemical-assisted methods 2+ →Fe 3+ This ensures that the effective oxidant concentration in the leaching system remains at a stable and sufficient level. Simultaneously, the superparamagnetic catalyst can be nearly 100% rapidly recovered and recycled using an external magnetic field. This not only reduces costs but also ensures consistently high catalytic activity in each leaching reaction. This core reagent and catalyst recycling mechanism avoids fluctuations or decreases in the leaching rate due to reagent consumption or catalyst deactivation, allowing the high recovery rate to be stably maintained during long-term operation.

[0061] Finally, for the recovery of precious metals, an optimized thiosulfate green leaching system was adopted, achieving efficient and selective leaching of gold and silver (recovery rate >96%) under mild conditions. This system, through the precise ratio of sodium thiosulfate, ammonia, ammonium sulfite, and copper sulfate, forms stable gold and silver complex anions under alkaline conditions. The addition of ammonium sulfite is crucial, effectively inhibiting the disproportionation decomposition of sodium thiosulfate and the formation of harmful byproducts (such as tetrathionate), thus maintaining the chemical stability and long-term effectiveness of the leaching system. The stable leaching environment avoids the re-passivation or ineffective consumption of gold and silver, ensuring near-complete dissolution. Simultaneously, since the preceding steps have efficiently removed most of the base metals, the interference from "impurities" during the gold and silver leaching process is minimal, further improving its leaching selectivity and efficiency.

[0062] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0063] Example 1 This embodiment provides a specific process for recycling rare and precious metals from waste circuit boards. The main metal contents of the raw materials processed are: tin 6.37%, copper 37.01%, aluminum 5.68%, nickel 0.22%, zinc 4.31%, gold 12.98 g / t, and silver 40.39 g / t. The process is implemented according to the following steps: S1, Preprocessing After manually disassembling waste circuit boards to remove large components, they are subjected to two-stage crushing using a shear crusher and a hammer crusher to obtain crushed material with a particle size of approximately 0.5 mm. This material is then placed in a microwave pyrolysis furnace and heated to 470°C at a microwave power of 1000 W / kg under nitrogen protection and held for 20 minutes. After pyrolysis, a dried pretreated material with an organic matter content of <0.5% is obtained.

[0064] S2, Alkaline Selective Tin Immersion The pretreated material was mixed with a 3 mol / L sodium hydroxide solution at a liquid-to-solid mass ratio of 5:1. The mixture was stirred and leached at 85°C for 2.5 hours. After leaching, the mixture was filtered to obtain a filtrate containing sodium stannate and a first filter residue. The tin-containing filtrate was sent to the tin recovery process, while the first filter residue was sent to the next step.

[0065] S3, catalytic synergistic leaching of main metals (1) Construction of leaching system: The first filter residue obtained from S2 is mixed with Fe... 3+The acidic leachate was mixed, and the liquid-to-solid mass ratio of the resulting slurry was 6:1. The leachate used was a sulfuric acid system, in which Fe... 3+ The concentration was 28 g / L, and the initial pH was adjusted to 1.8.

[0066] (2) Preparation and addition of catalyst: Magnetic nanocomposite catalyst-complexing agent was prepared according to the following method: a. Fe3O4 nanoparticles were synthesized using a hydrothermal method. Ferric chloride (FeCl3·6H2O) and ferrous sulfate (FeSO4·7H2O) were weighed at a molar ratio of 2.0:1, dissolved in deoxygenated water, and reacted at 180℃ for 10 h. The average particle size of the obtained Fe3O4 nanoparticles was 50 nm.

[0067] b. The above Fe3O4 nanoparticles, citric acid and urea were dispersed in water at a mass ratio of 1:3.5:2 and subjected to hydrothermal reaction at 190°C for 14 hours to form a composite material (Fe3O4@C-COOH / NH2) coated with a nitrogen-doped carbon shell rich in carboxyl and amino groups.

[0068] c. The composite material was dispersed in a 30 g / L aqueous solution of phosphotungstic acid and stirred at 40 °C for 6 h to immobilize the phosphotungstic acid on the carbon layer surface. The mass ratio of the composite material to phosphotungstic acid was 1:0.3. After immobilization, the intermediate was obtained by centrifugation.

[0069] d. The obtained intermediate was dispersed in a buffer solution at pH 5.0, and iminodiacetic acid (CAS No. 142-73-4) was added. The mass ratio of the intermediate to iminodiacetic acid was 1:0.5. The reaction was carried out at 50°C for 18 h with stirring, in the presence of the condensing agents 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDAC) and N-hydroxysuccinimide (NHS), wherein the molar ratio of EDAC to iminodiacetic acid was 1.2:1. The reaction resulted in the covalent grafting of iminodiacetic acid via amide bonds.

[0070] e. The above product was dispersed in ethanol, and 12% (by weight of the product) of hexadecyltrimethylammonium bromide (CTAB) was added. After stirring and impregnation at room temperature for 4 hours, the solvent was removed to obtain the final magnetic nanocomposite catalyst-complexing agent. The loading of phosphotungstic acid in the catalyst was determined to be 10 wt%, and the grafting amount of iminodiacetic acid was 5.5 wt%.

[0071] The prepared catalyst was added at a ratio of 1.0% of the total mass of the leaching system (leaching solution and first filter residue).

[0072] (3) Second leaching: turn on the ultrasonic field with a frequency of 40kHz and a power density of 100W / L, and start the electrochemical oxidation auxiliary device (the anode is a ruthenium-iridium coated titanium electrode), control the leaching system temperature at 55℃, and react for 3.5h.

[0073] S4, Separation and Diversion After leaching, an external permanent magnet is first used to rapidly separate and recover the magnetic nanocomposite catalyst-complexing agent from the slurry. The remaining slurry is then filtered to obtain a precious liquor containing copper, nickel, zinc, and aluminum ions, and a second filter residue rich in gold and silver. The recovered catalyst can be recycled after washing with dilute acid.

[0074] S5, Green Extracted Gold and Silver The second filter residue obtained from S4 was mixed with the gold leaching solution at a liquid-to-solid mass ratio of 6.5:1. The gold leaching solution contained the following components: sodium thiosulfate 0.22 mol / L, copper sulfate 0.04 mol / L, ammonia 1.4 mol / L, ammonium sulfite 0.10 mol / L, and trisodium citrate 0.06 mol / L. The pH of the system was adjusted to 10.0 with ammonia, and leaching was carried out at 60°C with stirring for 6 hours. After leaching, solid and liquid were separated to obtain a precious solution containing gold and silver and a harmless tailings mainly composed of silicon dioxide.

[0075] S6, Metal Recycling (1) Tin recycling: Carbon dioxide gas is bubbled into the tin-containing filtrate obtained in step S2 until the pH of the solution drops to approximately 7.5, causing the tin to precipitate completely in the form of metastannic acid. After filtration and thorough washing with deionized water, the precipitate is placed in a muffle furnace and calcined at 600°C for 2 hours to obtain a high-purity tin dioxide product. The mother liquor, after adding sodium hydroxide, can be recycled back to step S2.

[0076] (2) Separation and recovery of copper, nickel, zinc and aluminum: a. Copper Extraction and Electrowinning: The S4 precious solution was subjected to three-stage countercurrent extraction using 20% ​​LIX984N (kerosene solution) to selectively extract copper ions. The supported organic phase was then subjected to two-stage back-extraction with 180 g / L sulfuric acid solution to obtain a copper-rich electrolyte. This electrolyte was fed into an electrolytic cell and electrowinning was performed at a current density of 200 A / m² and a temperature of 50 °C to obtain a Grade A cathode copper plate.

[0077] b. Regeneration and diversion of iron solution: The residual liquid after copper extraction is pumped into the anode chamber of the diaphragm electrolytic cell, and the anode current density is controlled at 80 A / m. 2 The ferrous ions in the solution are completely oxidized to ferric ions to obtain regenerated Fe. 3+ Leaching agent. Most of the regenerated liquid is directly returned to step S3 for reuse as leachate, while a small portion is diverted to control impurity accumulation.

[0078] c. Stepwise precipitation of aluminum, zinc, and nickel: Sodium hydroxide solution is slowly added to the diverted solution while stirring. First, the pH is adjusted to 5.0 to completely precipitate aluminum as aluminum hydroxide. After filtration, washing, and calcination, alumina is obtained. Then, the pH of the filtrate is further adjusted to 8.0 to precipitate zinc as zinc hydroxide. The filter cake can be dissolved to prepare zinc salts or electrolyzed to obtain zinc. Finally, the pH of the solution is adjusted to above 10.0 to precipitate nickel as nickel hydroxide. The filter cake is then calcined and reduced to obtain nickel powder or used to prepare nickel salts.

[0079] (3) Enrichment and refining of gold and silver: a. Adsorption: The gold and silver-containing solution obtained from S5 is passed through an adsorption column packed with polyacrylamide-type anion exchange resin at a specific space velocity, and the thiosulfate complex anions of gold and silver are selectively adsorbed.

[0080] b. Desorption and Reduction: The saturated resin was desorbed using a mixed solution containing 0.8 mol / L thiourea and 1.0 mol / L hydrochloric acid. The eluent was collected, and an appropriate amount of sodium sulfite was added for reduction, causing gold and silver to precipitate as black powder.

[0081] c. Smelting: The gold and silver mixed powder obtained from precipitation is filtered, dried, and placed in a graphite crucible. An appropriate amount of borax is added as a flux, and smelting is carried out at 1200℃ to obtain gold and silver alloy ingots. The desorbed resin is regenerated with dilute sodium hydroxide solution, and the lean solution is returned to step S5 for recycling after replenishment with reagents.

[0082] The leaching rates and final recovery rates of all metals reached excellent levels. In the alkaline selective leaching in step S2, the leaching rate of tin reached 96.2%. In the catalytic synergistic leaching in step S3, the leaching rates of copper, aluminum, nickel, and zinc were 99.1%, 96.5%, 98.7%, and 97.8%, respectively. In the green leaching in step S5, the leaching rates of gold and silver were 97.5% and 98.0%, respectively.

[0083] Based on the high leaching rate mentioned above, and combined with subsequent efficient separation and purification steps, the final comprehensive recovery rates of each metal are as follows: tin recovery rate 95.1%, copper recovery rate 98.3%, aluminum recovery rate 94.8%, nickel recovery rate 97.2%, zinc recovery rate 96.0%, gold recovery rate 95.8%, and silver recovery rate 96.5%.

[0084] Under the same second leaching (S3) operating conditions, after 20 consecutive cycles of repeated use, the copper leaching rate was 99.1% in the first use and 95.8% in the 20th use, with an activity retention rate of 96.7%. After 20 cycles, ICP-OES (Inductively Coupled Plasma Atomic Emission Spectroscopy) analysis showed that the loading of phosphotungstic acid on the catalyst surface decreased from the initial 10.0 wt% to 9.2 wt%, and the grafting amount of iminodiacetic acid decreased from the initial 5.5 wt% to 5.0 wt%. The loss rate of active components was low.

[0085] To verify the structural stability of the magnetic nanocomposite catalyst-complexing agent in the second leaching acidic environment, 0.5 g of the magnetic nanocomposite catalyst-complexing agent was placed in 100 mL of simulated leaching solution (pH 1.8, Fe...). 3+ The catalyst was stirred at 55°C for 4 h in a sulfuric acid system with a concentration of 28 g / L. After the reaction, the catalyst was recovered by magnetic separation, and the iron ion concentration in the solution was determined by inductively coupled plasma atomic emission spectrometry (ICP-OES). The results showed that the iron ion concentration in the solution was 0.042 mg / L, and the calculated Fe dissolution rate was 0.084%. This indicates that the nitrogen-doped carbon layer effectively protects the Fe3O4 magnetic core in an acidic environment of pH 1.5–2.0, ensuring the catalyst structure is stable and preventing deactivation or iron ion contamination of the leachate due to the dissolution of the magnetic core.

[0086] Figure 2 This is a low-magnification TEM image of the magnetic nanocomposite catalyst-complexing agent provided in Example 1 of this application.

[0087] Depend on Figure 2 It can be seen that the magnetic nanocomposite catalyst-complexing agent exhibits an aggregated state of nanoparticles with an average particle size of approximately 50 nm.

[0088] Figure 3 This is a high-magnification TEM image of the magnetic nanocomposite catalyst-complexing agent provided in Example 1 of this application.

[0089] Depend on Figure 3 It can be seen that the magnetic nanocomposite catalyst-complexing agent exhibits a core-shell hierarchical structure, with a dense Fe3O4 magnetic core inside and an amorphous nitrogen-doped carbon layer with a thickness of about 2-5 nm uniformly coated on the outside. The carbon layer surface has uniformly distributed nanoscale protrusions and textures, which is the direct morphological manifestation after covalent grafting and physical loading of active components such as phosphotungstic acid and iminodiacetic acid.

[0090] Figure 4 The infrared spectrum of the magnetic nanocomposite catalyst-complexing agent provided in Example 1 of this application.

[0091] Depend on Figure 4It can be seen that the sample exhibited characteristic absorption peaks in the following wavenumber ranges, and the correspondence with the structural units of each functional component of the catalyst is clear: in ~580 cm⁻¹ -1 A medium-strong absorption peak appears at ~3400 cm⁻¹, attributed to the Fe-O stretching vibration of the Fe₃O₄ core; at ~3400 cm⁻¹... -1 A broad peak is observed at ~1250 cm⁻¹, which is attributed to the OH / NH stretching vibration. -1 A medium-intensity peak appears at ~1080 cm⁻¹, attributed to the CN stretching vibration. This peak position indicates that the carbon layer is rich in carboxyl and amino groups, confirming that the nitrogen-doped carbon layer has been successfully coated on the Fe₃O₄ surface. At ~1080 cm⁻¹... -1 and ~960cm -1 The presence of a moderately strong peak at ~1640 cm⁻¹ is attributed to the stretching vibrations of PO₄ and W=O terminal oxygen, respectively, clearly indicating that phosphotungstic acid has been successfully loaded onto the carbon layer surface; at ~1640 cm⁻¹... -1 and ~1540cm -1 The presence of a medium-intensity peak at ~2925 cm⁻¹ corresponds to the amide I band (C=O stretching) and the amide II band (NH bending), respectively, providing direct evidence that iminodiacetic acid is covalently linked to the carboxyl group of the carbon layer via an amide bond; at ~2925 cm⁻¹... -1 and ~2850cm -1 The presence of medium-intensity peaks at these locations, attributed to the asymmetric / symmetric stretching vibrations of CH2 and CH3 respectively, confirms that hexadecyltrimethylammonium bromide has been effectively coated onto the catalyst surface. Thus, the infrared spectroscopy data comprehensively confirms at the molecular level the successful construction of each functional component of the magnetic nanocomposite catalyst-complexer.

[0092] Example 2 This embodiment provides a process for recycling rare and precious metals from waste circuit boards. The main metal contents of the raw materials processed are: tin 7.02%, copper 35.44%, aluminum 5.91%, nickel 0.31%, zinc 3.87%, gold 13.75 g / t, and silver 38.92 g / t. The process is implemented according to the following steps: S1. Pretreatment: Place the material crushed to a particle size of about 0.4 mm in a microwave pyrolysis furnace, and heat it to 450℃ with a microwave power of 800W / kg under nitrogen protection and keep it at that temperature for 30 minutes.

[0093] S2, Alkaline Selective Tin Immersion: The pretreated material is mixed with a 2.5 mol / L sodium hydroxide solution at a liquid-to-solid mass ratio of 4:1, and stirred and immersed at 90°C for 3 hours.

[0094] S3. Catalytic synergistic leaching of the main metal: The first filter residue obtained in S2 was mixed with an acidic leaching solution with a Fe³⁺ concentration of 25 g / L at a liquid-to-solid mass ratio of 5:1, with an initial pH of 2.0. 0.8% of a magnetic nanocomposite catalyst-complexing agent (preparation parameters were the same as in Example 1) was added. The reaction was carried out at 50°C for 4 h under an ultrasonic field with a frequency of 30 kHz and a power density of 80 W / L, assisted by electrochemical oxidation.

[0095] S4. Separation and diversion: After magnetic separation to recover the catalyst, solid-liquid separation is performed to obtain the precious liquid and the second filter residue.

[0096] S5. Green leaching of gold and silver: The second filter residue is mixed with the gold leaching system solution (sodium thiosulfate 0.18mol / L, copper sulfate 0.03mol / L, ammonia water 1.0mol / L, ammonium sulfite 0.08mol / L, trisodium citrate 0.04mol / L) at a liquid-solid mass ratio of 7:1, the pH is adjusted to 9.8, and leaching is carried out at 58℃ for 7 hours.

[0097] S6. Metal Recovery and Results: The leaching rate of tin was determined to be 96.8%; the leaching rates of copper, aluminum, nickel, and zinc were 94.7%, 95.9%, 98.2%, and 97.2%, respectively; and the leaching rates of gold and silver were 96.9% and 97.5%, respectively. The final comprehensive recovery rates were: tin 95.9%, copper 97.8%, aluminum 93.5%, nickel 96.5%, zinc 95.1%, gold 94.6%, and silver 95.8%.

[0098] Example 3 This embodiment provides a process for recycling rare and precious metals from waste circuit boards. The main metal contents of the raw materials processed are: tin 5.85%, copper 41.23%, aluminum 6.88%, nickel 0.41%, zinc 4.65%, gold 16.42 g / t, and silver 45.31 g / t. The process is implemented according to the following steps: S1. Pretreatment: Place the material crushed to a particle size of about 0.7mm in a microwave pyrolysis furnace, and heat it to 500℃ with a microwave power of 1300W / kg under nitrogen protection and keep it at that temperature for 15min.

[0099] S2, Alkaline Selective Tin Immersion: The pretreated material is mixed with a 3.5 mol / L sodium hydroxide solution at a liquid-to-solid mass ratio of 6:1, and stirred and immersed at 82°C for 2 hours.

[0100] S3. Catalytic synergistic leaching of the main metal: The first filter residue obtained in S2 was mixed with an acidic leaching solution with a Fe³⁺ concentration of 30 g / L at a liquid-to-solid mass ratio of 7:1, with an initial pH of 1.6. 1.2% of the total mass of a magnetic nanocomposite catalyst-complexing agent (preparation parameters were the same as in Example 1) was added. The reaction was carried out at 58°C for 2.5 h under an ultrasonic field with a frequency of 45 kHz and a power density of 120 W / L, assisted by electrochemical oxidation.

[0101] S4. Separation and diversion: After magnetic separation to recover the catalyst, solid-liquid separation is performed to obtain the precious liquid and the second filter residue.

[0102] S5. Green leaching of gold and silver: The second filter residue is mixed with the gold leaching system solution (sodium thiosulfate 0.28mol / L, copper sulfate 0.05mol / L, ammonia water 1.8mol / L, ammonium sulfite 0.12mol / L, trisodium citrate 0.08mol / L) at a liquid-solid mass ratio of 5:1, the pH is adjusted to 10.3, and leaching is carried out at 62℃ for 5 hours.

[0103] S6. Metal Recovery and Results: The leaching rate of tin was determined to be 96.5%; the leaching rates of copper, aluminum, nickel, and zinc were 99.4%, 97.1%, 99.0%, and 98.3%, respectively; and the leaching rates of gold and silver were 98.1% and 98.5%, respectively. The final comprehensive recovery rates were: tin 94.9%, copper 98.8%, aluminum 95.6%, nickel 97.8%, zinc 97.1%, gold 96.4%, and silver 97.2%.

[0104] Comparative Example 1 This comparative example is modified from the one disclosed in Example 1 as follows: In step S3 (catalytic synergistic leaching of the host metal), no magnetic nanocomposite catalyst-complexing agent is added; only Fe-containing catalysts are used. 3+ The acidic leachate was leached under the same ultrasonic field and electrochemical oxidation conditions, and the remaining steps and parameters were exactly the same as in Example 1.

[0105] Metal recovery performance: After treatment, the leaching rate of tin was 96.0%, and the final recovery rate was 94.9%, similar to Example 1. The leaching rates of gold and silver were 97.3% and 97.8%, respectively, with final recovery rates of 95.5% and 96.2%, also similar to Example 1. However, due to the lack of catalytic oxidation-complexation synergy, the leaching of the main metals was significantly inhibited, with leaching rates of copper, aluminum, nickel, and zinc at only 85.2%, 78.6%, 82.1%, and 80.4%, respectively. After subsequent separation processes, the final overall recovery rates were: copper 84.1%, aluminum 75.8%, nickel 80.5%, and zinc 78.9%.

[0106] Comparative Example 2 This comparative example is modified from the one disclosed in Example 1 as follows: In step S3, during the preparation of the magnetic nanocomposite catalyst-complexing agent, urea was not added; only citric acid was used for hydrothermal carbonization. The remaining steps and parameters were exactly the same as in Example 1.

[0107] Metal recovery results: Tin leaching rate 96.1%, final recovery rate 94.8%; gold leaching rate 97.4%, final recovery rate 95.6%; silver leaching rate 98.1%, final recovery rate 96.4%. Due to the lack of urea, an effective nitrogen-doped carbon layer could not be formed, resulting in a lack of amino functional groups on the carbon shell surface. This not only reduced the subsequent loading and stability of phosphotungstic acid but also severely affected the covalent grafting of iminodiacetic acid. The synergistic function of the catalyst's oxidation-complexation was greatly weakened. Therefore, the leaching rates of copper, aluminum, nickel, and zinc were significantly reduced to 87.5%, 80.2%, 84.9%, and 82.7%, respectively. Their final comprehensive recovery rates were: copper 86.3%, aluminum 77.1%, nickel 83.4%, and zinc 80.5%.

[0108] Comparative Example 3 This comparative example is modified from the one disclosed in Example 1 as follows: In step S3, during the preparation of the magnetic nanocomposite catalyst-complexing agent, phosphotungstic acid is not added. The remaining steps and parameters are exactly the same as in Example 1.

[0109] Metal recovery efficiency: Tin leaching rate 96.3%, final recovery rate 95.0%; Gold leaching rate 97.6%, final recovery rate 95.7%; Silver leaching rate 98.0%, final recovery rate 96.3%. Because the catalyst completely lacks phosphotungstic acid oxidation centers, it loses its interfacial catalytic oxidation ability and relies solely on Fe in the solution. 3+ Oxidation occurs, and the complexation effect of IDA cannot be effectively utilized due to the lack of adjacent oxidation products. The leaching kinetics of the main metals are extremely slow. The leaching rates of copper, aluminum, nickel, and zinc are 83.6%, 75.4%, 79.8%, and 78.1%, respectively, with final overall recoveries of 82.4% for copper, 72.1% for aluminum, 78.3% for nickel, and 76.0% for zinc.

[0110] Comparative Example 4 This comparative example is modified from the one disclosed in Example 1 as follows: In step S3, during the preparation of the magnetic nanocomposite catalyst-complexing agent, iminodiacetic acid is not added. The remaining steps and parameters are exactly the same as in Example 1.

[0111] Metal recovery efficiency: Tin leaching rate 95.9%, final recovery rate 94.7%; gold leaching rate 97.2%, final recovery rate 95.3%; silver leaching rate 97.9%, final recovery rate 96.1%. Due to the lack of iminodiacetic acid complexation sites in the catalyst, it only possesses the oxidation function of phosphotungstic acid. The dissolved metal ions cannot be captured and removed in time, and are prone to redeposition or hydrolysis on the particle surface to form a passivation layer, severely hindering the continued reaction. Therefore, the leaching rates of copper, aluminum, nickel, and zinc are 89.1%, 82.3%, 86.7%, and 84.5%, respectively, with final comprehensive recovery rates of: copper 87.9%, aluminum 79.0%, nickel 85.2%, and zinc 82.3%.

[0112] Comparative Example 5 This comparative example is modified from the one disclosed in Example 1 as follows: In step S5, copper sulfate was not added to the gold immersion solution. The remaining steps and parameters were exactly the same as in Example 1.

[0113] Metal recovery performance: The leaching rates and final recovery rates of tin and the main metals (copper, aluminum, nickel, and zinc) were basically consistent with those in Example 1. Due to the lack of copper sulfate in the gold leaching system, the crucial copper-ammonia complex ion catalytic oxidation center could not be formed, resulting in extremely slow and incomplete oxidation and dissolution processes for gold and silver. The leaching rates of gold and silver decreased significantly to 68.5% and 72.3%, respectively, with final overall recovery rates of only 66.8% and 70.1%, respectively.

[0114] Comparative Example 6 This comparative example is modified from the one disclosed in Example 1 as follows: Ammonium sulfite was not added to the gold leaching solution in step S5. The remaining steps and parameters were exactly the same as in Example 1.

[0115] Metal recovery performance: The leaching rates and final recovery rates of tin and the main metals (copper, aluminum, nickel, and zinc) were basically consistent with those in Example 1. Due to the lack of the stabilizer ammonium sulfite in the gold leaching system, sodium thiosulfate rapidly underwent disproportionation decomposition under alkaline conditions and copper ammonia ion catalysis, generating harmful byproducts such as polythionates. This made the leaching system extremely unstable and quickly ineffective. The leaching rates of gold and silver were severely affected, at 75.2% and 78.9%, respectively, with final comprehensive recovery rates of 73.4% and 76.5%, respectively.

[0116] Comparative Example 7 This comparative example is modified from the one disclosed in Example 1 as follows: In step S5, no ammonia was added to the gold immersion solution, and sodium hydroxide was used to maintain the pH of the system at 10.0. The remaining steps and parameters were exactly the same as in Example 1.

[0117] Metal recovery performance: The leaching rates and final recovery rates of tin and the main metals (copper, aluminum, nickel, and zinc) were basically consistent with those in Example 1. Due to the lack of ammonia in the gold leaching system, it was unable to form a catalytically active copper-ammonia complex with copper ions, and it also could not effectively stabilize thiosulfate ions. The redox potential of the system was imbalanced, and the decomposition of sodium thiosulfate was accelerated. The leaching rates of gold and silver decreased significantly, to 71.8% and 76.4%, respectively, with final comprehensive recovery rates of 70.0% and 74.2%, respectively.

[0118] The leaching rates of tin, copper, aluminum, nickel, zinc, gold, and silver in Examples 1-3 and Comparative Examples 1-7 are summarized in Table 1.

[0119] Table 1. Leaching rates of tin, copper, aluminum, nickel, zinc, gold, and silver As shown in Table 1, Examples 1 to 3 exhibited excellent and stable leaching effects on various metals. The leaching rate of tin remained at a high level of 96.2%–96.8%; the leaching rates of the main metals copper, aluminum, nickel, and zinc ranged from 94.7% to 99.4%, 95.9% to 97.1%, 98.2% to 99.0%, and 97.2% to 98.3%, respectively; and the leaching rates of the precious metals gold and silver also reached 96.9%–98.1% and 97.5%–98.5%, respectively.

[0120] In Comparative Example 1, due to the complete absence of the magnetic nanocomposite catalyst-complexing agent, the leaching rate of the main metals (copper, aluminum, nickel, and zinc) dropped significantly to below 85.2%, proving that the catalyst is indispensable for achieving efficient leaching.

[0121] Comparative Examples 2 to 4, by respectively omitting key raw materials (urea, phosphotungstic acid, and iminodiacetic acid) in catalyst preparation, revealed the core roles of each functional component of the catalyst. The absence of any component weakened its oxidation-complexation synergistic function, resulting in a significantly lower main metal leaching rate compared to the examples, but the recovery of precious metals remained unaffected.

[0122] Comparative Examples 5 to 7, by respectively omitting key components (copper sulfate, ammonium sulfite, and ammonia) in the gold leaching system, resulted in a sharp drop in gold and silver leaching rates to below 76.4%, while the recovery of the main metal remained unaffected. This demonstrates that the thiosulfate gold leaching system requires the synergistic effect of its components to maintain a stable oxidation-complexation environment; the absence of any component will lead to system failure.

[0123] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0124] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A process for recycling rare and precious metals from waste circuit boards, characterized in that, The process includes the following steps: S1. Disassemble and crush the waste circuit boards, and perform microwave pyrolysis pretreatment to obtain pretreated materials; S2. The pretreated material is subjected to a first leaching with a strong alkaline solution, and after solid-liquid separation, a tin-containing filtrate and a first filter residue are obtained. S3. The first filter residue containing Fe 3+ The acidic leachate and the magnetic nanocomposite catalyst-complexing agent were mixed, and then a second leaching was carried out under the assistance of ultrasonic field and electrochemical oxidation to obtain the leachate. The magnetic nanocomposite catalyst-complexer comprises: a magnetic core, a nitrogen-doped carbon layer coated on the surface of the magnetic core, and an active component loaded on the nitrogen-doped carbon layer, wherein the active component comprises: phosphotungstic acid, iminodiacetic acid, and a quaternary ammonium salt cationic surfactant. S4. The magnetic nanocomposite catalyst-complexing agent is recovered by magnetic separation, followed by solid-liquid separation to obtain a precious liquid containing copper, nickel, zinc and aluminum and a second filter residue. S5. The second filter residue is mixed with the gold leaching system solution for a third leaching. After solid-liquid separation, a precious solution containing gold and silver is obtained. The gold leaching system solution includes: sodium thiosulfate, copper sulfate, ammonia, ammonium sulfite and trisodium citrate. S6. Recover the corresponding metals from the tin-containing filtrate, the precious solution containing copper, nickel, zinc, and aluminum, and the precious solution containing gold and silver, respectively.

2. The process for recycling rare and precious metals from waste circuit boards according to claim 1, characterized in that, The particle size of the pretreated material is 0.3–0.8 mm; The microwave pyrolysis pretreatment is carried out under an inert atmosphere at a temperature of 450–500°C, a holding time of 15–30 min, and a microwave power of 600–1500 W / kg.

3. The process for recycling rare and precious metals from waste circuit boards according to claim 1, characterized in that, The strong alkaline solution is a sodium hydroxide solution with a concentration of 2-4 mol / L; The first leaching includes the following parameters: temperature of 80-95℃, time of 2-3h, and liquid-solid ratio of (4-6):

1.

4. The process for recycling rare and precious metals from waste circuit boards according to claim 1, characterized in that, The containing Fe 3+ In the acidic leachate, Fe 3+ The concentration is 25–30 g / L; The first filter residue and the Fe-containing 3+ The liquid-to-solid ratio of the acidic leachate mixture is (4-8):1; The second leaching process includes the following parameters: initial pH value of 1.5 to 2.0, temperature of 50 to 60°C, and time of 2 to 4 hours; The frequency of the ultrasonic field is 20–50 kHz, and the power density is 50–150 W / L.

5. The process for recycling rare and precious metals from waste circuit boards according to claim 1, characterized in that, The added mass of the magnetic nanocomposite catalyst-complexer is the mass containing Fe. 3+ The acidic leachate accounts for 0.5 to 1.5% of the total mass of the first filter residue.

6. The process for recycling rare and precious metals from waste circuit boards according to claim 5, characterized in that, The preparation method of the magnetic nanocomposite catalyst-complexing agent includes the following steps: Fe3O4 nanoparticles were obtained by dissolving ferric salts and ferrous salts in water and carrying out a hydrothermal reaction at 160–200 °C for 6–12 h under an inert atmosphere. The Fe3O4 nanoparticles were dispersed in an aqueous solution of citric acid and urea, and then subjected to a hydrothermal reaction at 180–200 °C for 8–16 h to form a nitrogen-doped carbon shell rich in carboxyl and amino groups on the surface of the Fe3O4 nanoparticles, thus obtaining the Fe3O4@C-COOH / NH2 composite material. The Fe3O4@C-COOH / NH2 composite material was dispersed in an aqueous solution of phosphotungstic acid and stirred at 30–50 °C for 4–8 h to immobilize the phosphotungstic acid on the surface of the carbon layer, thus obtaining an intermediate supported by phosphotungstic acid. The intermediate was dispersed in a buffer solution, and iminodiacetic acid was added. In the presence of condensing agents 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide, the mixture was stirred at pH 4.5–5.5 and 40–60°C for 12–24 h to allow the amino group of iminodiacetic acid to covalently graft onto the carboxyl group on the surface of the carbon layer through an amidation reaction, thereby obtaining a complexed functionalized intermediate. The complexed functionalized intermediate was dispersed in ethanol, and a quaternary ammonium salt cationic surfactant was added. After stirring and impregnation at room temperature for 2-6 hours, the solvent was removed so that the quaternary ammonium salt cationic surfactant modified the surface of the complexed functionalized intermediate, thereby obtaining the magnetic nanocomposite catalyst-complexing agent.

7. The process for recycling rare and precious metals from waste circuit boards according to claim 6, characterized in that, The molar ratio of the trivalent ferric salt to the divalent ferric salt is (1.5–2.5):1; The mass ratio of the Fe3O4 nanoparticles, the citric acid and the urea is 1:(2-5):(1-3); The concentration of the phosphotungstic acid aqueous solution is 10–50 g / L, and the mass ratio of the Fe3O4@C-COOH / NH2 composite material to phosphotungstic acid is 1:(0.1–0.5). The mass ratio of the intermediate to iminodiacetic acid is 1:(0.2-0.8), and the molar ratio of 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride to iminodiacetic acid is (1-1.5):1; The quaternary ammonium salt cationic surfactant is hexadecyltrimethylammonium bromide, and the mass of hexadecyltrimethylammonium bromide is 5-20% of the mass of the complexed functionalized intermediate.

8. The process for recycling rare and precious metals from waste circuit boards according to claim 6, characterized in that, In the magnetic nanocomposite catalyst-complexing agent, the Fe3O4 nanoparticles have a particle size of 30-80 nm, the phosphotungstic acid loading is 5-15 wt%, and the iminodiacetic acid grafting amount is 3-8 wt%.

9. The process for recycling rare and precious metals from waste circuit boards according to claim 1, characterized in that, In the gold immersion system, the concentrations of each component are as follows: sodium thiosulfate 0.15–0.30 mol / L, copper sulfate 0.02–0.06 mol / L, ammonia 0.8–2.0 mol / L, ammonium sulfite 0.05–0.15 mol / L, and trisodium citrate 0.02–0.10 mol / L.

10. The process for recycling rare and precious metals from waste circuit boards according to claim 1, characterized in that, The third leaching process includes the following parameters: liquid-to-solid ratio of (5-8):1, pH value of 9.5-10.5, temperature of 55-65℃, and leaching time of 4-8h.

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