How does the gold elution process work?

The gold elution process is a key step in the recovery of gold from activated carbon after it has been adsorbed during the cyanidation process. Elution involves removing gold from the loaded activated carbon so it can be recovered in subsequent processes such as electrowinning. Here’s how the process generally works:

1. Overview of the Gold Elution Process

Gold elution separates gold adsorbed onto activated carbon. Gold cyanide ions (Au(CN)₂⁻) are chemically desorbed from the carbon in a high-temperature and high-pressure environment, often using a solution of caustic soda (sodium hydroxide) and cyanide.

2. Steps in the Elution Process

The elution process typically follows these stages:

• The activated carbon loaded with gold is washed to remove fine impurities and residual solids.
• Washing improves the efficiency during the elution process by ensuring a clean surface on the carbon for effective desorption of gold.

• The loaded carbon is placed in an elution column.
• A heated eluent solution (commonly sodium hydroxide (NaOH) and cyanide (NaCN)) is pumped through the column under high temperature (usually 110–120°C) and pressure (up to 3–5 bar).
• This solution strips the gold from the carbon. Heat enhances the desorption process by weakening the bond between the gold cyanide complex and the carbon.
• The chemical reaction can be summarized as follows:\[\text{Au(CN)₂⁻ (adsorbed on carbon) → Au(CN)₂⁻ (in solution)}.\]
• The gold-rich solution produced during this process is known as the eluate.

• The eluate exits the column containing high concentrations of gold dissolved as the gold cyanide complex (Au(CN)₂⁻).
• The eluate is typically sent to an electrowinning circuit for gold recovery, where an electric current is applied to plate the gold onto cathodes.

3. Regeneration of Activated Carbon:

After the elution process, the activated carbon is no longer loaded with gold but may still contain impurities. To prepare it for reuse:

• The carbon undergoes reactivation in a rotary kiln or furnace at high temperatures (e.g., 600–650°C) to remove organic contaminants and regain its adsorption properties.

4. Common Elution Methods

There are several elution techniques, each tailored to different operating conditions:

• Uses a solution of 0.1%–0.2% NaOH and 0.1% NaCN.
• Operates at 95°C–100°C at atmospheric pressure.
• Slower process, typically 10–14 hours for completion.

• Uses a more concentrated eluent preparation and operates at higher temperatures (100°C–120°C) under pressurized conditions.
• Faster process, typically 8–12 hours.
• Requires a preg-robbing pre-treatment for organic matter prior to elution.

• Operates at higher temperatures (140°C–160°C) and pressures (up to 5 bar).
• More efficient than traditional Zadra and AARL processes.
• Produces concentrated eluate in shorter timeframes.

5. Factors Affecting the Elution Process:

• Temperature and Pressure: Higher temperatures accelerate gold desorption but may degrade the carbon if excessive.
• Concentration of Eluent: Proper amounts of cyanide and caustic soda are essential to prevent incomplete stripping.
• Carbon Fines and Fouling: Presence of fines or organic fouling impairs elution efficiency.
• Stroomsterkte: Maintaining optimal flow rate ensures uniformly distributed eluent and minimizes dead zones.

6. Gold Recovery from Eluate:

The eluate (solution from the elution process) is processed by elektrowinning, where an electric current is applied to deposit solid gold onto steel or cathodes. The gold is then smelted into doré bars.

7. Environmental and Safety Considerations:

• Caustic soda and cyanide are hazardous materials requiring careful handling and disposal.
• The process must comply with environmental regulations to minimize impacts on ecosystems and groundwater.

In summary, the gold elution process utilizes chemical and thermal processes to desorb gold from activated carbon, so it can be recovered through electrowinning. Various methods, such as Zadra or AARL, are optimized based on operational requirements, balancing efficiency, safety, and environmental stewardship.

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