How does zinc powder replacement recover gold concentrate?

2025-10-04

Zinc powder replacement, commonly referred to as the Merrill-Crowe process, is a widely used method in gold recovery that involves precipitating gold (and sometimes silver) from a solution of gold-cyanide complexes using zinc powder. Here’s an overview of how zinc powder replacement works:

1. Formation of Gold-Cyanide Complex

• Gold concentrate is dissolved in a cyanide solution during the leaching process, typically using sodium cyanide (NaCN) as a leaching agent.
• The cyanide reacts with gold to form a stable gold-cyanide complex:
  \[\text{4Au} + \text{8CN}^- + \text{O}_2 + \text{2H}_2\text{O} \rightarrow \text{4Au(CN)}_2^- + \text{4OH}^-\]

2. Clarification and Deoxygenation

• To prepare the solution for gold recovery, it is first clarified to remove any suspended solids to ensure clear liquid.
• Deoxygenation is then performed by passing the gold-cyanide solution through a vacuum tower to reduce dissolved oxygen. This step is critical because oxygen can interfere with the next step by re-dissolving gold.

3. Zinc Powder Precipitation

• The deoxygenated solution is mixed with zinc powder, which triggers a chemical reaction where gold is precipitated out of solution.
• Zinc acts as a more reactive metal and replaces gold in the cyanide complex:\[2Au(CN)_2^- + Zn \rightarrow 2Au + Zn(CN)_4^{2-}\]
• In this displacement reaction, gold precipitates out as metallic gold, while the zinc forms a zinc-cyanide complex that remains in solution.

4. Filtration and Recovery

• The precipitated gold is separated from the liquid through filtration or other solid-liquid separation methods. The solid residue contains the metallic gold, which is further refined to high purity.

5. Refining

• The recovered gold is subjected to smelting or other refining processes to produce pure gold bullion or concentrate.

Advantages of Zinc Powder Replacement

• Selective Recovery: Zinc powder specifically targets gold and silver, leaving other dissolved metals behind.
• High Efficiency: The process can achieve high recovery rates for gold and silver.
• Scalable: The Merrill-Crowe process is suitable for large-scale operations.

Factors Affecting Efficiency

Several factors can influence the effectiveness of the zinc powder replacement process, including:

1. Deoxygenation: Complete removal of oxygen ensures maximum precipitation efficiency.
2. Zinc Powder Quality: Fine zinc powder increases reaction rate and gold recovery.
3. Solution pH: The cyanide solution is typically maintained at a pH of 10–11 to prevent cyanide degradation.
4. Concentration of Metals: Higher concentrations of gold in the solution improve recovery efficiency.

In summary, the Merrill-Crowe process utilizes zinc’s higher reactivity to displace gold from the solution, enabling recovery and further refining of gold concentrate.

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