What Is The Method For Extracting Gold From E-waste With An Eco-friendly Gold Extractant And Detecting The Cyanide Concentration?

Gold Recovery from E-Waste Using Eco-Friendly Extraction Reagents

I. Pretreatment Steps

1.1 Crushing and Screening

Purpose: Increase surface area to facilitate subsequent gold leaching.
Operations:
① Use a crusher to break down e-waste (e.g., circuit boards, CPUs, gold fingers) into 0.5–1 mm particles.
② Screen the material to remove oversized or undersized particles, ensuring uniform particle size.
③ Employ magnetic separation to remove ferromagnetic impurities (e.g., iron, nickel).
④ Rinse the crushed material with clean water to eliminate dust and impurities, then air-dry for further use.

1.2 Roasting Treatment (Optional)

Purpose: Remove organic materials and break the bonding between metals and plastics.
Operations:
① Place the crushed e-waste in a roasting furnace and roast at 500–600°C for 1–2 hours.
② Ensure proper ventilation during roasting to prevent the accumulation of harmful gases.
③ After roasting, allow the waste to cool to room temperature, then perform secondary crushing until the particle size is less than 0.5 mm.

II. Preparation of Eco-Friendly Gold Extraction Agent YX500 Solution

2.1 Preparation of Eco-Friendly Gold Extraction Agent YX500 Solution

Reagent: Eco-friendly gold extraction agent YX500.
Concentration: Prepare a YX500 solution with a concentration of 0.05%–0.1% (i.e., 0.5–1 g/L).
Method:
① Add an appropriate amount of clean water into the mixing tank.
② Slowly add the eco-friendly gold extraction agent YX500 in proportion while continuously stirring until it is completely dissolved.
③ Dosing time: Ensure the operation is completed within 10–20 minutes.

2.2 Alkalinity Adjustment

Purpose: Prevent hydrogen cyanide gas volatilization and ensure smooth leaching reaction.
Operations:
① Add sodium hydroxide (NaOH) or lime milk to adjust the solution pH to 10–11.
② Use pH test strips or a pH meter to verify the solution's alkalinity reaches the appropriate level.

III. Leaching Process

3.1 Leaching Equipment

Equipment: Tower leaching tank or mechanically agitated tank.
Temperature:

Ambient temperature (20–25°C).

If leaching acceleration is required, temperature may be increased to 40–50°C.

3.2 Reagent Addition & Reaction Conditions

Dosing sequence:
① First, add sodium hydroxide (NaOH) solution for pH adjustment.
② Then, add the pre-prepared eco-friendly gold extraction agent YX500 solution and start the stirring device.
③ Dosing time: Must be completed within 10–20 minutes.
Stirring speed: 200–300 rpm to ensure full contact between materials and solution.

3.3 Leaching Time & Oxidant Usage

Leaching time:

At ambient temperature: 24–48 hours.

At 40–50°C: Can be reduced to 12–24 hours.
Oxidant:
① To accelerate gold dissolution, hydrogen peroxide (H₂O₂, 0.1–0.5%) may be added or air may be introduced.
② Addition timing: Synchronized with the YX500 solution dosing and maintained continuously.

IV. Solid-Liquid Separation

Filtration and Washing

Method: Vacuum filtration or centrifugal separation equipment shall be employed.
Operations:
① Filter the leached slurry to separate the gold-bearing solution (pregnant solution) from the residue.
② Wash the residue with dilute alkaline solution (pH 10-11) to recover residual gold elements.

V. Gold Recovery Methods

Method 1: Zinc Powder Replacement Process

Steps:
① Slowly add zinc powder to the pregnant solution at a ratio of 5-10 g/L.
② Maintain continuous stirring with a reaction time of 2-4 hours.
③ Filter to obtain gold mud.

Method 2: Electrolysis Process

Equipment: Stainless steel cathode, graphite or lead anode.
Conditions:
① Current density: 1-2 A/dm², Voltage: 2-3 V.
② Electrolysis duration: 6-12 hours.
Operations:
① After energizing the electrolytic cell, gold gradually deposits on the cathode.
② Remove the cathode and scrape off the deposited gold mud.

VI. Gold Mud Treatment and Refinement

Acid Washing and Smelting

Steps:
① Use dilute nitric acid or aqua regia to dissolve impurities, followed by filtration to obtain purified gold mud.
② Place the gold mud in a high-temperature electric furnace for smelting, then cast into gold ingots.
Purity: Can reach ≥99.9%.

VII. Waste Liquid Treatment and Environmental Protection Measures

Compliant Discharge

Testing: Verify cyanide concentration to ensure it remains below 0.2 mg/L.
Discharge: After meeting standards, release into wastewater treatment system.

VIII. Safety Precautions

① Ventilation: Maintain adequate ventilation in work areas to prevent hydrogen cyanide gas accumulation.
② Protection: Operators must wear gloves, masks, and protective goggles to ensure safety.
③ First Aid: Prepare amyl nitrite and other antidotes for emergency treatment of cyanide poisoning.

Detection of Cyanide Ion (CN¯) Concentration in Eco-Friendly Gold Extraction Reagents

Testing the cyanide ion (CN¯) concentration in eco-friendly gold extraction agents is a critical step to ensure their safety and effectiveness. The following outlines commonly used detection methods and their key operational points, categorized into two main types: laboratory testing methods and on-site rapid testing methods.

I. Laboratory Precision Detection Methods

1.1 Silver Nitrate Titration (Classical Method)

Principle: Cyanide ions react with silver nitrate to form soluble [Ag(CN)₂]¯ complexes, with excess silver ions reacting with an indicator (e.g., silver chromate) to produce a color change.
Steps:
① Dilute the sample and add sodium hydroxide (pH >11) to prevent hydrogen cyanide (HCN) volatilization.
② Use silver chromate as an indicator and titrate with standardized silver nitrate solution until the color changes from yellow to orange-red.
Scope: Suitable for high cyanide concentrations (>1 mg/L); provides precise results but requires laboratory conditions.

1.2 Spectrophotometry (Isonicotinic Acid-Pyrazolone Method)

Principle: In weakly acidic conditions, cyanide reacts with chloramine-T to form cyanogen chloride (CNCl), which then reacts with isonicotinic acid-pyrazolone to produce a colored compound. Quantification is achieved by measuring absorbance at 638 nm.
Steps:
① Distill the sample if necessary to remove interferents.
② Add buffer and chromogenic reagents, then measure absorbance using a spectrophotometer. Calculate concentration via a standard curve.
Advantage: High sensitivity (detection limit: 0.001 mg/L), ideal for trace-level analysis.

1.3 Ion-Selective Electrode (ISE) Method

Principle: A cyanide electrode responds to CN¯ activity, measuring concentration via potential difference.
Steps:
① Adjust sample pH to >12 with NaOH to avoid HCN interference.
② Calibrate the electrode, measure potential, and convert to concentration.
Advantage: Rapid operation, broad detection range (0.1–1000 mg/L), but requires regular electrode calibration.

II. On-Site Rapid Detection Methods

2.1 Rapid Test Strips

Principle: Strips contain chromogenic agents (e.g., picric acid) that change color (yellow to reddish-brown) upon reaction with cyanide ions.
Procedure: Immerse the strip in the sample, then compare the color against a reference card for semi-quantitative reading.
Features: Highly portable but relatively low accuracy; suitable for emergency screening.

2.2 Portable Cyanide Detectors

Principle: Miniaturized spectrophotometric or electrode-based devices (e.g., Hach, Merck).
Operation: Direct sample injection with automatic concentration display.
Advantage: Combines speed and high precision, ideal for field use in mining areas.

2.3 Pyridine-Barbituric Acid Colorimetry (Simplified)

Reagent Kit: Pre-packaged tubes with chromogenic agents; add water sample for colorimetric analysis.
Detection Limit: ~0.02 mg/L, suitable for low-cyanide testing in eco-friendly gold extraction agents.

III. Precautions

Safety Measures

Cyanide is highly toxic! All testing must be conducted in a fume hood to prevent skin contact or inhalation.

Waste liquid treatment: Oxidize with sodium hypochlorite (CN¯ + ClO¯ → CNO¯ + Cl¯).

Interference Factors

Sulfide (S²¯) and heavy metal ions may cause interference. Pre-distillation or masking agents (e.g., EDTA) should be used to eliminate their effects.

Method Selection

High-precision testing: Laboratory titration or spectrophotometry is preferred.

Rapid screening: Test strips or portable devices are more practical.