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.

Prominer (Shanghai) Mining Technology Co., Ltd. specializes in providing complete mineral processing and advanced materials solutions globally. Our core focus includes: gold processing, lithium ore beneficiation, industrial minerals. Specializing in anode material production and graphite processing.

Products include: Grinding & Classification, Separation & Dewatering, Gold Refining, Carbon/Graphite Processing and Leaching Systems.

We offer end-to-end services including engineering design, equipment manufacturing, installation, and operational support, backed by 24/7 expert consultation.

Our Website Url: https://www.prominetech.com/
Our Email: [email protected]
Our Sales: +8613918045927(Richard), +8617887940518(Jessica), +8613402000314(Bruno)