I. Pre-treatment steps
1. Crushing and Screening
Purpose:
Increase surface area to improve subsequent gold leaching efficiency.
Procedure:
Crushing:
Use a crusher or shredder to break down e-waste (such as discarded PCBs, CPUs, gold fingers, etc.) into 0.5–1 mm particles.
Screening:
Pass the crushed material through a vibrating sieve to remove oversized or undersized fragments, ensuring uniform particle size distribution.
Magnetic Separation:
Remove ferrous metals (iron, nickel) using a belt-type magnetic separator or overband magnet.
Washing & Drying:
Rinse the processed material with clean water to eliminate dust and impurities.
Air-dry or oven-dry (at 60–80°C) before further processing.
2. Roasting Treatment (Optional)
Purpose:
Remove organic materials (plastics, resins, adhesives).
Break bonds between metals and non-metallic components.
Procedure:
Roasting Process:
Load the pre-crushed e-waste into a roasting furnace.
Heat at 500–600°C for 1–2 hours in a controlled oxygen-limited environment (to prevent excessive oxidation of metals).
Ventilation & Safety:
Maintain active exhaust ventilation (e.g., fume extraction system) to avoid accumulation of toxic gases (e.g., dioxins, furans).
Use a scrubber or afterburner if required by environmental regulations.
Post-Roasting Treatment:
Cool the roasted material to room temperature in a sealed container (to prevent re-absorption of moisture).
Re-crush the brittle residue to <0.5 mm particles (using a ball mill or mortar) to enhance subsequent leaching efficiency.
II. Preparation of YX500 Solution
1. Solution Formulation
Reagent: YX500
Concentration: Prepare a 0.05%–0.1% (0.5–1 g/L) YX500 solution.
Procedure:
Water Addition:
Fill a mixing tank with an appropriate volume of clean water (preferably deionized or distilled).
Reagent Dosing:
Gradually add YX500 powder/liquid into the tank while stirring to ensure complete dissolution.
Mixing Time:
Control the dosing process within 10–20 minutes to achieve homogeneous dispersion.
2. Alkalinity Adjustment
Purpose:
Prevent hydrogen cyanide (HCN) gas volatilization by maintaining alkaline conditions.
Ensure stable gold leaching reaction efficiency.
Procedure:
pH Adjustment:
Add sodium hydroxide (NaOH) solution or lime slurry (Ca(OH)₂) to the leaching system.
Gradually adjust the pH to 10–11 (optimal range for cyanide leaching).
pH Monitoring:
Verify alkalinity using pH test strips (for quick checks) or a calibrated pH meter (for precision).
Stir thoroughly after reagent addition to ensure uniform pH distribution.
III. Leaching Process
1. Leaching Equipment
Equipment: Column leaching tank or mechanically agitated reactor.
Temperature:
Ambient (20–25°C) for standard operation.
Optional heating (40–50°C) to accelerate leaching kinetics.
2. Reagent Dosing & Reaction Conditions
Dosing Sequence:
First, add sodium hydroxide (NaOH) solution to adjust pH to 10–11.
Then, introduce the prepared YX500 solution and start the agitator.
Dosing duration: Complete within 10–20 minutes.
Agitation: Maintain 200–300 rpm to ensure uniform contact between solids and solution.
3. Leaching Time & Oxidant Use
Leaching Duration:
Ambient temp. (20–25°C): 24–48 hours.
Heated (40–50°C): Can be reduced to 12–24 hours.
Oxidant Application (Optional):
To enhance gold dissolution, add hydrogen peroxide (H₂O₂, 0.1–0.5%) or introduce air sparging.
Timing: Add oxidant concurrently with YX500 solution, maintaining continuous dosing.
IV. Solid-Liquid Separation
Filtration & Washing
Method: Vacuum filtration or centrifugal separation.
Procedure:
Primary Filtration:
Filter the leached slurry using a vacuum filter or centrifuge to separate:
Gold-bearing solution (pregnant liquor)
Residue (tailings)
Residue Washing:
Rinse the residue with dilute alkaline solution (pH 10–11) to recover residual gold.
Repeat washing (2–3 cycles) until effluent tests negative for gold.
V. Gold Recovery Methods
Method 1: Zinc Powder Cementation (Zinc Precipitation)
Procedure:
Zinc Addition:
Slowly add zinc powder (5–10 g/L) to the pregnant gold solution under agitation.
Reaction:
Maintain stirring for 2–4 hours to ensure complete gold displacement.
Filtration:
Filter the slurry to collect gold sludge (Au + excess Zn).
Method 2: Electrolysis (Electrowinning)
Equipment:
Cathode: Stainless steel plate/mesh.
Anode: Graphite or lead alloy.
Operating Conditions:
Current density: 1–2 A/dm².
Voltage: 2–3 V.
Duration: 6–12 hours.
Procedure:
Electrodeposition:
Pass current through the electrolytic cell; gold deposits on the cathode.
Recovery:
Remove the cathode and scrape off the gold sludge.
Comparison of Methods
| Method | Advantages | Disadvantages |
| Zinc Powder | – Fast, low-cost | – High zinc consumption |
| – Simple equipment | – Impurities (Zn, Pb) in gold sludge | |
| Electrolysis | – High-purity gold (>99%) | – Slow, energy-intensive |
| – Reusable cathodes | – Sensitive to solution composition |
Safety Reminder:
Zinc method: Perform in ventilated areas (H₂ gas byproduct).
Electrolysis: Avoid chloride ions to prevent chlorine gas generation.
VI. Gold Sludge Treatment and Refining
1. Acid Washing & Smelting
Procedure:
Acid Leaching (Impurity Removal):
Treat the gold sludge with dilute nitric acid (HNO₃) or aqua regia (HCl:HNO₃ = 3:1) to dissolve base metals (Cu, Zn, Pb).
Filter and rinse with deionized water to isolate purified gold solids.
Smelting:
Transfer the purified gold sludge to a graphite crucible.
Melt in a high-temperature furnace (≥1100°C) with a flux (e.g., borax/soda ash) to remove residual oxides.
Pour the molten gold into a mold to cast 99.9%+ pure gold ingots.
VII. Waste Liquid Treatment & Environmental Protection Measures
Compliance Discharge
Procedure:
Cyanide Concentration Testing:
Analyze wastewater using ion-selective electrodes (ISE) or titration methods (e.g., ASTM D2036).
Ensure free cyanide (CN⁻) concentration < 0.2 mg/L (meets international effluent standards).
Treatment for Discharge:
If over-limit, apply chemical oxidation:
Hydrogen peroxide (H₂O₂) + catalyst (e.g., Cu²⁺) to break down cyanide to CO₂ + N₂.
Alkaline chlorination (NaOCl at pH ≥ 10) for complete detoxification.
Discharge treated water into industrial wastewater systems after confirmation.
VIII. Safety Precautions
1. Ventilation
Requirement:
Operate in a well-ventilated fume hood or local exhaust system to prevent hydrogen cyanide (HCN) gas accumulation.
Install HCN gas detectors with audible alarms (threshold: ≥1 ppm).
2. Personal Protective Equipment (PPE)
Mandatory Gear:
Chemical-resistant gloves (nitrile or neoprene).
Full-face respirator with cyanide-rated cartridges (e.g., 3M™ MultiGas Cartridge).
Splash goggles and impervious lab coat/apron.
3. Emergency First Aid
Cyanide Antidote Kit:
Amyl nitrite ampoules (inhaled for 30 seconds every minute).
Sodium thiosulfate (IV) + hydroxocobalamin (for severe exposure).
Protocol:
Immediate evacuation of the victim to fresh air.
Administer antidote per MSDS guidelines.
Call emergency services (provide cyanide exposure details).
