Cyanide-free gold leaching purification process for waste circuit board

Abstract

The application belongs to the technical field of noble metal purification, and specifically discloses a cyanide-free gold leaching and purification process for waste circuit boards, which comprises the following steps: crushing the waste circuit boards, low-temperature passivation, deionized water washing, screening and draining for standby; adopting histidine-glycine composite complexing leaching solution with pH 7.2-7.8, constant-temperature leaching at 45-55 DEG C for 3-6 hours, filtering and separating the gold-containing leaching solution; adding activated carbon into the gold-containing leaching solution, pressure filtration, and obtaining gold-containing mother liquor; adding zinc powder into the mother liquor, stirring and replacing, standing and settling after replacement is completed, filtering and collecting black gold mud; and placing the black gold mud in a nitric acid solution for impurity removal, roasting and smelting ingot casting, and obtaining high-purity gold ingot. The cyanide-free gold leaching and purification process for waste circuit boards has low copper, nickel and iron base metal leaching rate, reduces impurity dissolution from the source, reduces refining cost, and is superior to the conventional amino acid gold leaching process.

Description

Technical Field

[0001] This invention relates to the field of precious metal purification technology, and in particular to a cyanide-free gold immersion purification process for waste circuit boards. Background Technology

[0002] Waste circuit boards are the material with the highest concentration of precious metals among electronic waste. The gold content in each ton of waste circuit boards can reach 150~300g, which is far higher than the grade of natural gold ore, and has extremely high resource recycling value. At present, the mainstream gold extraction processes in industry are divided into two categories: cyanidation method and cyanide-free wet gold extraction: (1) Cyanidation method: The technology is mature and the gold leaching rate is high, but cyanide is highly toxic. There are risks of leakage, poisoning and environmental pollution in the production process. Environmental protection control is strict and does not meet the requirements of green solid waste recycling development; (2) Conventional cyanide-free process: The mainstream is the thiosulfate method, thiourea method and chlorination method. The thiosulfate system has poor stability, large reagent consumption and serious interference from copper impurities; the thiourea method leaches gold under acidic conditions, which seriously corrodes the copper and nickel metal of the circuit board substrate, with many by-products and great difficulty in purification; the chlorination method is strongly acidic and oxidizing, which seriously corrodes the equipment, causes great waste gas pollution and high operation and maintenance costs.

[0003] The cyanide-free leaching gold technology generally suffers from three major pain points: First, the leaching system has extreme acidity and alkalinity, with strong acids and alkalis corroding the equipment and damaging the valuable substrate metal of the circuit board; second, the single complexing agent has poor selectivity, with copper, iron, and nickel impurities being leached out simultaneously, increasing the difficulty of subsequent purification; third, the leaching agent has weak stability, with agent decomposition and loss during the reaction process, resulting in large fluctuations in the leaching rate; and fourth, the purification process is cumbersome, with low purity of gold mud and high refining costs.

[0004] To address the aforementioned technical problems, this invention develops a low-corrosion, high-selectivity, and reagent-stable cyanide-free gold leaching purification process, filling the technological gap in neutral, mild, cyanide-free gold leaching and low-impurity purification. Summary of the Invention

[0005] The purpose of this invention is to provide a cyanide-free gold leaching purification process for waste circuit boards. This process employs a composite amino acid complex system, combined with a weakly oxidizing synergist, to achieve selective gold leaching under neutral and mild conditions, reducing substrate metal corrosion and impurity leaching. It optimizes the parameters of the entire leaching, enrichment, and refining process, improving gold recovery rate and finished product purity. Simultaneously, it solves the problems of high reagent consumption and poor stability in traditional cyanide-free processes, balancing environmental friendliness with the economic viability of industrial production.

[0006] To achieve the above objectives, the present invention provides a cyanide-free gold immersion purification process for waste circuit boards, comprising the following steps: Step 1, Material pretreatment: Waste circuit boards are crushed in two stages to a particle size of 1.5~2.0mm, passivated at low temperature, rinsed with deionized water, screened to remove non-metallic impurities, and drained for later use; Step 2, Neutral cyanide-free gold leaching: Histidine-glycine complex leaching solution is used with a liquid-to-solid ratio of 4.5:1, pH 7.2~7.8, and constant temperature leaching at 45-55℃ for 3-6 hours. The gold-containing leaching solution is then filtered and separated. Step 3, purification and impurity removal: Add activated carbon to the gold-containing leaching solution to adsorb impurities, filter under pressure, and obtain pure gold-containing mother liquor; Step 4, zinc powder directional replacement: Add 200~300 mesh atomized zinc powder to the mother liquor, stir and replace at 30-35℃ for 1.5-4 hours, let stand and settle after replacement, filter and collect black gold mud; Step 5, refining and purification: The black gold mud is placed in a nitric acid solution to remove impurities, roasted, smelted and cast into ingots to obtain high-purity gold ingots.

[0007] Preferably, in step 1, two-stage crushing is used: the first stage coarse crushing has a particle size of 8~10mm, and the second stage fine crushing has a particle size of 1.5~2.0mm, to preserve the metal bonding state and avoid excessive crushing that could lead to the introduction of impurities.

[0008] Preferably, in step 1, low-temperature passivation involves placing the crushed material in a hot air passivation furnace at a temperature of 115°C for 30-60 minutes to remove the epoxy resin organic coating and flux residue from the circuit board surface.

[0009] Preferably, in step 2, the leachate includes histidine, glycine, pyridinecarboxylic acid, hydrogen peroxide, sodium bicarbonate, anhydrous sodium sulfate, and deionized water.

[0010] Preferably, the concentrations of each component in the leachate are as follows: histidine 0.85 mol / L, glycine 0.40 mol / L, pyridinecarboxylic acid 0.06 mol / L; hydrogen peroxide 4.5 wt%; sodium bicarbonate 2.2 wt%; anhydrous sodium sulfate 0.5 wt%; and the remainder is deionized water.

[0011] Preferably, in step 2, the stirring speed is 220 r / min, an external DC micro electric field is applied, the electrode spacing is 12 cm, the constant voltage is 2.4 V, and the oxidation-reduction potential of the system is controlled to be stable at +320~+360 mV. After leaching, vacuum filtration is performed to separate the gold-containing leaching solution from the solid waste residue. The waste residue is treated to be harmless, and the gold-containing filtrate is retained for later use.

[0012] Preferably, in step 4, weakly acidic deionized water with pH=5.2-5.6 is used to rinse the black gold mud to remove residual impurities.

[0013] Preferably, in step 5, the mass fraction of the nitric acid solution is 12%, the solid-liquid ratio of the nitric acid solution to the black gold mud is 3:1, the temperature is 60-70℃, and the soaking is carried out at a constant temperature for 80-100 minutes to remove impurities.

[0014] Preferably, in step 5, the roasting process involves washing the gold mud after impurity removal until it is neutral and then drying it, followed by roasting it in a muffle furnace at 680°C for 40-60 minutes to remove impurities.

[0015] Preferably, in step 5, smelting and casting the ingot: borax is added as a flux, the smelting temperature is 1180℃, the melting is held at this temperature for 20-35 minutes, and the ingot is naturally cooled and demolded to obtain a high-purity gold ingot.

[0016] The advantages and beneficial effects of the above-mentioned cyanide-free gold immersion purification process for waste circuit boards in this invention are as follows: 1. This invention relies on the complexation mechanism of multiple amino acids and the passivation mechanism of base metals, combined with precise potential control, to achieve extremely low leaching rates of base metals such as copper, nickel, and iron. This reduces the leaching of impurities from the source, lowers refining costs, and is superior to conventional amino acid leaching gold processes.

[0017] 2. The buffer system of this invention maintains a constant pH, has a low reagent loss rate, and the leachate can be recycled and reused. It adopts a hydrogen peroxide-sodium bicarbonate weak oxidation buffer system with micro-electric field-assisted regulation to precisely fix the redox potential at +320~+360mV, precisely and selectively oxidizes elemental gold, and at the same time forms a trace amount of complex passivation film on the surface of base metals such as copper, nickel, and iron, which greatly inhibits the dissolution of base metals. The selectivity is far superior to conventional cyanide-free reagents.

[0018] 3. This invention employs a combined purification process of electric field enhanced displacement, low-temperature nitric acid fractionation for impurity removal, and vacuum melting and refining. It eliminates the need for organic extractants and features online impurity monitoring, resulting in a stable gold purity of 99.99% in the finished product, enhancing industrial continuity. This invention constructs a synergistic mechanism of "amino acid coordination complexation, base metal surface passivation, and electric field-directed enrichment," which distinguishes it from traditional physical leaching and simple complexation leaching processes at the reaction principle level.

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] Figure 1 This is a bar chart showing the immersion gold ratio of the embodiments and comparative examples of the present invention; Figure 2 The bar chart shows the drug loss rate of the embodiments and comparative examples of the present invention; Figure 3 The bar chart shows the impurity leaching rate of the embodiments and comparative examples of the present invention. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0023] The following examples are not intended to limit the invention, but are only for illustration. Unless otherwise specified, the experimental methods used in the following examples are generally performed under conventional conditions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0024] Example 1 A cyanide-free gold immersion purification process for waste circuit boards employs a composite amino acid complex system, combined with a weak oxidizing synergist, to achieve selective gold immersion under neutral and mild conditions. The process includes the following steps: Step 1, Material Pre-treatment Process: (1) Raw materials: discarded computer motherboards with a gold content of 185~220g / t.

[0025] (2) Graded crushing: Two-stage crushing is adopted. The first stage coarse crushing has a particle size of 8mm, and the second stage fine crushing has a particle size of 1.8mm. The metal bonding state is preserved to avoid excessive crushing and impurities from being mixed in.

[0026] (3) Low-temperature passivation to remove impurities: Place the crushed material in a hot air passivation furnace at a temperature of 115°C and a holding time of 45 minutes to remove the epoxy resin organic coating and flux residue on the surface of the circuit board.

[0027] (4) Washing and screening: Rinse twice with deionized water, screen to remove powdery non-metallic impurities, drain and set aside. The moisture content of the material is ≤5%.

[0028] Step 2, Neutral Composite Cyanide-Free Immersion Gold Process: (1) Preparation of leachate (combination of molar concentration and mass concentration): histidine 0.85mol / L, glycine 0.40mol / L, pyridine carboxylic acid 0.06mol / L (multi-component complex synergistic system); hydrogen peroxide 4.5% (oxidant); sodium bicarbonate 2.2% (pH buffer); anhydrous sodium sulfate 0.5% (conductive additive); the remainder is deionized water.

[0029] (2) Liquid-solid ratio: The liquid-solid ratio of the leachate to the pretreated circuit board material is 4.5:1 (mL / g).

[0030] (3) Reaction environment: pH value is constant at 7.5 (neutral), leaching temperature is 48℃, stirring speed is 220r / min, external DC micro electric field is applied, electrode spacing is 12cm, constant voltage is 2.4V, and the oxidation-reduction potential (ORP) of the system is controlled to be stable at +340mV.

[0031] (4) Leaching time: constant temperature, sealed, stirring and leaching for 3.5 hours.

[0032] (5) Post-treatment: After leaching, vacuum filtration is performed using a 200-mesh filter cloth to separate the gold-containing leachate from the solid waste residue. The waste residue is treated to render it harmless, and the gold-containing filtrate is stored for later use.

[0033] Step 3, Leachate purification and impurity removal process: (1) Adsorption and removal of impurities: Add modified activated carbon (coconut shell activated carbon, particle size 1 mm) to the gold-containing leaching solution at an amount of 2.5 g / L, stir at room temperature for 55 min to adsorb and remove trace amounts of copper and iron organic impurities in the solution.

[0034] (2) Precision filtration: a 0.22μm organic filter membrane is used for pressure filtration to remove suspended solid particles and obtain pure gold-containing mother liquor with a gold ion concentration of 55~62mg / L.

[0035] Step 4, Targeted Replacement Gold Extraction Process: (1) Displacement agent: atomized zinc powder (particle size 250 mesh, purity ≥99.5%).

[0036] (2) Process parameters: The amount of zinc powder added is 1.8 times the theoretical gold replacement mass, the replacement temperature is 32℃, the stirring speed is 150r / min, and the replacement reaction time is 2.0h.

[0037] (3) Solid-liquid separation: After the replacement is completed, let it stand for 40 minutes to settle, filter and collect the black gold mud, and rinse the gold mud 3 times with weak acid deionized water (pH=5.5) to remove residual zinc salt impurities.

[0038] Step 5, refining and purifying ingot casting process: (1) Acid leaching to remove impurities: Place the gold mud in a dilute nitric acid solution (mass fraction 12%), with a liquid-to-solid ratio of 3:1 and a temperature of 65℃, and leach for 90 minutes to remove base metal impurities such as zinc, copper, and nickel.

[0039] (2) Drying and roasting: After removing impurities, the gold mud is washed until neutral, dried at 105℃ for 2 hours, and then placed in a muffle furnace at 680℃ for 45 minutes to remove organic residues and carbon impurities.

[0040] (3) Melting and casting ingots: Add 0.3% borax as flux, melt at 1180℃, keep warm for 25 minutes, cool naturally and demold to obtain high-purity gold ingots.

[0041] Example 2 A cyanide-free gold immersion purification process for waste circuit boards includes the following steps: Step 1, Material Pre-treatment Process: (1) Raw materials: mobile phone circuit boards with a gold content of 190~210g / t.

[0042] (2) Graded crushing: Two-stage crushing is adopted. The first stage coarse crushing has a particle size of 10mm, and the second stage fine crushing has a particle size of 1.5mm. The metal bonding state is preserved to avoid excessive crushing and impurities from being mixed in.

[0043] (3) Low-temperature passivation to remove impurities: Place the crushed material in a hot air passivation furnace at a temperature of 115°C and a holding time of 60 minutes to remove the epoxy resin organic coating and flux residue on the surface of the circuit board.

[0044] (4) Washing and screening: Rinse with deionized water 3 times, screen to remove powdery non-metallic impurities, drain and set aside. The moisture content of the material is ≤5%.

[0045] Step 2, Neutral Composite Cyanide-Free Immersion Gold Process: (1) Preparation of leachate (combination of molar concentration and mass concentration): histidine 0.85mol / L, glycine 0.40mol / L, pyridine carboxylic acid 0.06mol / L (multi-component complex synergistic system); hydrogen peroxide 4.5% (oxidant); sodium bicarbonate 2.2% (pH buffer); anhydrous sodium sulfate 0.5% (conductive additive); the remainder is deionized water.

[0046] (2) Liquid-solid ratio: The liquid-solid ratio of the leachate to the pretreated circuit board material is 4.5:1 (mL / g).

[0047] (3) Reaction environment: pH value is constant at 7.3 (neutral), leaching temperature is 45℃, stirring speed is 220r / min, external DC micro electric field is applied, electrode spacing is 12cm, constant voltage is 2.4V, and the oxidation-reduction potential (ORP) of the system is controlled to be stable at +360mV.

[0048] (4) Leaching time: 5 hours of constant temperature, sealed and stirred leaching.

[0049] (5) Post-treatment: After leaching, vacuum filtration is performed using a 200-mesh filter cloth to separate the gold-containing leachate from the solid waste residue. The waste residue is treated to render it harmless, and the gold-containing filtrate is stored for later use.

[0050] Step 3, Leachate purification and impurity removal process: (1) Adsorption and removal of impurities: Add modified activated carbon (coconut shell activated carbon, particle size 1.2 mm) to the gold-containing leaching solution at a dosage of 2.7 g / L, stir at room temperature for 45 min to adsorb and remove trace amounts of copper and iron organic impurities in the solution.

[0051] (2) Precision filtration: a 0.22μm organic filter membrane is used for pressure filtration to remove suspended solid particles and obtain pure gold-containing mother liquor with a gold ion concentration of 50-60mg / L.

[0052] Step 4, Targeted Replacement Gold Extraction Process: (1) Displacement agent: atomized zinc powder (particle size 200 mesh, purity ≥99.5%).

[0053] (2) Process parameters: The amount of zinc powder added is 1.8 times the theoretical gold replacement mass, the replacement temperature is 35℃, the stirring speed is 150r / min, and the replacement reaction time is 3h.

[0054] (3) Solid-liquid separation: After the replacement is completed, let it stand for 40 minutes to settle, filter and collect the black gold mud, and rinse the gold mud 3 times with weak acid deionized water (pH=5.3) to remove residual zinc salt impurities.

[0055] Step 5, refining and purifying ingot casting process: (1) Acid leaching to remove impurities: Place the gold mud in a dilute nitric acid solution (mass fraction 12%), with a liquid-to-solid ratio of 3:1 and a temperature of 70℃, and leach for 100 minutes to remove zinc, copper, and nickel base metal impurities.

[0056] (2) Drying and roasting: After removing impurities, the gold mud is washed until neutral, dried at 105℃ for 2 hours, and then placed in a muffle furnace at 680℃ for 45 minutes to remove organic residues and carbon impurities.

[0057] (3) Melting and casting ingots: Add 0.3% borax as flux, melt at 1180℃, keep warm for 35 minutes, cool naturally and demold to obtain high-purity gold ingots.

[0058] Example 3 Waste mobile phone circuit boards from an electronic solid waste treatment plant, with a gold content of 206 g / t and a total material weight of 10 kg, were selected and processed according to the process parameters described in Example 1: 1. Crushing and passivation: After two-stage crushing, the material particle size is 1.8mm. Passivation is carried out at 115℃ for 45min to remove the surface resin coating. The non-metallic impurity removal rate is 96.3%.

[0059] 2. Leaching reaction: Prepare 45L of composite leaching solution, turn on micro electric field assistance, constant voltage 2.4V, constant temperature 48℃ and stir for 3.5h. After vacuum filtration, obtain gold-containing leaching solution. The gold leaching rate is 99.36% and the copper leaching rate is only 0.87%.

[0060] 3. Purification and replacement: Add 2.5g / L of modified activated carbon to adsorb and remove impurities, filter, add atomized zinc powder for replacement, and let it settle to obtain black gold mud.

[0061] 4. Refining and casting ingots: After removing impurities with dilute nitric acid and roasting at high temperature, the ingots are smelted to obtain 1.985g of gold ingots with a purity of 99.99% and a total gold recovery rate of 97.82%.

[0062] Comparative Example 1 Unlike Example 1, the leachate does not include glycine and pyridine carboxylic acid; otherwise, it is the same as in Example 1.

[0063] Comparative Example 2 Unlike Example 1, the leachate does not contain histidine and pyridine carboxylic acid, but the rest is the same as in Example 1.

[0064] Comparative Example 3 Unlike Example 1, the leachate does not contain histidine; otherwise, it is the same as Example 1.

[0065] Comparative Example 4 Unlike Example 1, the leachate does not contain sodium bicarbonate and has a pH of 6.1; otherwise, it is the same as Example 1.

[0066] The processes of Example 1 and Comparative Examples 1-4 were tested, and the test results are shown in Table 1.

[0067] Table 1 Test Results

[0068] Comparing Comparative Example 1 and Comparative Example 2: Both histidine and glycine, as single amino acids, have obvious shortcomings. Glycine has the weakest complexing ability, followed by histidine, which proves that single amino acids cannot meet the requirements of high leaching rate and low impurities.

[0069] Comparative Example 1: Glycine alone possesses only a simple amino coordination structure, resulting in an extremely low stability constant for gold complexation and an inability to stably lock in gold ions. Furthermore, it lacks passivating components, leading to high base metal dissolution and easy hydrolysis loss of the reagent, resulting in extremely poor gold leaching ability. Comparative Example 3: Pyridine carboxylic acid can achieve metal passivation and activate the gold surface, thus exhibiting a better copper dissolution rate than glycine alone (e.g., ...). Figure 3 (as shown); however, the lack of histidine containing an imidazole ring as the main ligand results in insufficient overall complexation capacity, low upper limit of gold dissolution, and the gold leaching rate is still far lower than that of the ternary system.

[0070] Comparing Comparative Examples 2 and 3: After adding pyridine carboxylic acid, the selectivity of the system was significantly improved and the copper leaching rate decreased, confirming that pyridine carboxylic acid has unique functions of base metal passivation and gold surface activation; however, high gold leaching rate could not be achieved by combining it with glycine alone.

[0071] Comparing Comparative Example 3 with Example 1: Based on a system containing glycine and pyridine carboxylic acid, the introduction of histidine to construct a ternary system increased the gold leaching rate from 76.54% to 98.93% (e.g., ...). Figure 1 As shown), the drug loss rate decreased significantly (e.g. Figure 2 As shown in the figure, this indicates that the three are not simply physically mixed, but rather produce a synergistic coordination coupling chemical reaction, forming a more stable multi-component chelate structure.

[0072] Comparative Example 4: Sodium bicarbonate buffer can stabilize the pH of the system and maintain the amino acid complexation activity. Without buffer, the pH of the system fluctuates, and the gold immersion efficiency decreases significantly.

[0073] Example 1: The synergistic coordination effect of compound amino acids is far superior to that of single amino acids. The ratio of two amino acids significantly improves the complexing ability of gold and reduces drug loss.

[0074] This invention employs a multi-component complex system of histidine-glycine composite amino acids and trace amounts of pyridine carboxylic acid synergist to replace conventional single and binary amino acid complex formulations. By utilizing the synergistic coordination of two amino acids and the microscopic activation effect of heterocyclic carboxylic acids, the activation energy of the gold surface reaction is reduced, and gold is stably dissolved under neutral, room temperature, and low pressure conditions, completely avoiding corrosion from strong acids and alkalis. This is different from the common single amino acid immersion gold solutions on the market.

[0075] By employing a hydrogen peroxide-sodium bicarbonate weak oxidation buffer system combined with micro-electric field-assisted regulation, the oxidation-reduction potential (ORP) is precisely fixed at +320~+360mV, enabling precise and selective oxidation of elemental gold. Simultaneously, a trace amount of complex passivation film is formed on the surfaces of copper and nickel base metals, greatly inhibiting the dissolution of base metals. The selectivity is far superior to conventional cyanide-free reagents.

[0076] Adding a pretreatment low-temperature passivation and graded crushing process removes organic coatings and reactive impurities from the circuit board surface, reducing reagent consumption.

[0077] This invention employs a combined purification process involving electric field-enhanced displacement, low-temperature nitric acid fractionation for impurity removal, and vacuum melting refining. It eliminates the need for organic extractants and features online impurity monitoring, resulting in a stable gold purity of 99.99% and enhanced industrial continuity. The invention establishes a synergistic mechanism of "amino acid coordination complexation, base metal surface passivation, and electric field-directed enrichment," distinguishing it from traditional physical leaching and simple complexation leaching processes at the reaction principle level. Histidine, as the main ligand, provides strong complexing ability; glycine improves solution wetting and permeability; and pyridine carboxylic acid achieves gold surface activation and selective base metal passivation. These three components form a ternary synergistic coupling effect, jointly achieving high leaching rates and low impurity dissolution under neutral conditions.

[0078] Therefore, this invention employs the aforementioned cyanide-free gold leaching purification process for waste circuit boards, utilizing a composite amino acid complex system combined with a weakly oxidizing synergistic agent to achieve selective gold leaching under neutral and mild conditions, reducing substrate metal corrosion and impurity leaching; optimizing the parameters of the entire process of leaching, enrichment, and refining to improve gold recovery rate and finished product purity, while solving the problems of high reagent consumption and poor stability in traditional cyanide-free processes, thus balancing environmental friendliness and economic efficiency for industrial production.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A cyanide-free gold immersion purification process for waste circuit boards, characterized in that, Includes the following steps: Step 1, Material pretreatment: Waste circuit boards are crushed in two stages to a particle size of 1.5~2.0mm, passivated at low temperature, rinsed with deionized water, screened to remove non-metallic impurities, and drained for later use; Step 2, Neutral cyanide-free gold leaching: Histidine-glycine complex leaching solution is used with a liquid-to-solid ratio of 4.5:1, pH 7.2~7.8, and constant temperature leaching at 45-55℃ for 3-6 hours. The gold-containing leaching solution is then filtered and separated. Step 3, purification and impurity removal: Add activated carbon to the gold-containing leaching solution to adsorb impurities, filter under pressure, and obtain pure gold-containing mother liquor; Step 4, zinc powder directional replacement: Add 200~300 mesh atomized zinc powder to the mother liquor, stir and replace at 30-35℃ for 1.5-4 hours, let stand and settle after replacement, filter and collect black gold mud; Step 5, refining and purification: The black gold mud is placed in a nitric acid solution to remove impurities, roasted, smelted and cast into ingots to obtain high-purity gold ingots.

2. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 1, two-stage crushing is used: the first stage coarse crushing has a particle size of 8~10mm, and the second stage fine crushing has a particle size of 1.5~2.0mm. This preserves the metal-bonded state and avoids excessive crushing that could lead to the introduction of impurities.

3. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 1, low-temperature passivation involves placing the crushed material in a hot air passivation furnace at 115°C for 30-60 minutes to remove the epoxy resin organic coating and flux residue from the circuit board surface.

4. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 2, the leachate includes histidine, glycine, pyridine carboxylic acid, hydrogen peroxide, sodium bicarbonate, anhydrous sodium sulfate, and deionized water.

5. The cyanide-free gold immersion purification process for waste circuit boards according to claim 4, characterized in that: The concentrations of each component in the leachate were as follows: histidine 0.85 mol / L, glycine 0.40 mol / L, pyridinecarboxylic acid 0.06 mol / L; hydrogen peroxide 4.5 wt%; sodium bicarbonate 2.2 wt%; anhydrous sodium sulfate 0.5 wt%; and the remainder was deionized water.

6. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 2, the stirring speed is 220 r / min, an external DC micro electric field is applied, the electrode spacing is 12 cm, and the constant voltage is 2.4 V. The oxidation-reduction potential of the system is controlled to be stable at +320~+360 mV. After leaching, vacuum filtration is performed to separate the gold-containing leachate from the solid waste residue. The waste residue is treated to render it harmless, and the gold-containing filtrate is retained for later use.

7. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 4, weakly acidic deionized water with pH 5.2-5.6 is used to rinse the black gold mud and remove residual impurities.

8. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 5, the mass fraction of nitric acid solution is 12%, the solid-liquid ratio of nitric acid solution to black gold mud is 3:1, the temperature is 60-70℃, and the soaking is carried out at a constant temperature for 80-100 minutes to remove impurities.

9. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 5, the roasting process involves washing the gold mud after impurity removal until it is neutral and then drying it. It is then placed in a muffle furnace at 680°C for 40-60 minutes to remove impurities.

10. The cyanide-free gold immersion purification process for waste circuit boards according to claim 1, characterized in that: In step 5, smelting and casting the ingot: borax is added as a flux, the smelting temperature is 1180℃, the melting is held at this temperature for 20-35 minutes, and the ingot is naturally cooled and demolded to obtain a high-purity gold ingot.

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