Gold ore is primarily processed using the cyanide process, which involves grinding the ore and treating it with a cyanide solution to extract the gold.
The processing of gold ore is a multi-step procedure designed to liberate the valuable gold particles from the surrounding rock. While various methods exist, the cyanide process stands out as the principal technology used globally for this purpose.
The Principal Processing Technology: The Cyanide Process
The cyanide process is a hydrometallurgical technique where gold is chemically dissolved from the ore. This method is effective because gold forms soluble complex ions in the presence of cyanide and oxygen.
Key Steps in the Cyanide Process
Processing gold ore via the cyanide method typically follows these main stages:
- Comminution (Grinding): The first crucial step is comminution, which involves crushing and grinding the gold ore into smaller particles. This process significantly increases the surface area of the ore. Increasing the surface area is essential as it exposes the embedded gold particles to the leaching solution, allowing for more efficient extraction.
- Practical Insight: The degree of grinding depends on the nature of the ore, particularly how finely the gold is disseminated within the rock.
- Extraction (Leaching): After comminution, the ore is subjected to the extraction phase, which is conducted by treatment with a solution of cyanide. In this step, the ground ore comes into contact with a dilute alkaline cyanide solution (typically sodium cyanide, potassium cyanide, or calcium cyanide). Oxygen is also required for the chemical reaction to occur. The gold dissolves into the solution, forming a soluble gold-cyanide complex.
- Chemical Reaction (Simplified): 4 Au + 8 CN⁻ + O₂ + 2 H₂O → 4 [Au(CN)₂]⁻ + 4 OH⁻
- Examples of Cyanide Solutions: Sodium cyanide (NaCN) is commonly used.
The extraction, or leaching, can be carried out using different methods depending on the characteristics of the ore and the scale of the operation.
Leaching Methods
The extraction is primarily conducted using two common methods:
- Dump Leaching: This method is typically used for low-grade ores. The ore is placed in heaps, and the cyanide solution is applied to the top, percolating downwards through the ore and dissolving the gold. The gold-bearing solution is then collected at the base of the dump.
- Heap Leaching: Similar to dump leaching but often applied to slightly higher-grade ores or ores specifically crushed for leaching. The ore is stacked on an impermeable pad, and the cyanide solution is dripped or sprayed onto the heap. The pregnant solution flows down and is collected for gold recovery.
| Processing Step | Purpose | Method/Medium Involved |
|---|---|---|
| Comminution | Increase surface area, expose gold | Crushing, Grinding |
| Extraction (Leaching) | Dissolve gold from ore | Solution of cyanide (e.g., NaCN) |
| Leaching Method | Application method for cyanide solution to ore | Dump Leaching, Heap Leaching |
Following the leaching step, the gold-bearing solution is processed further to recover the gold, often using techniques like carbon adsorption or Merrill-Crowe precipitation, although these steps are beyond the scope of the initial extraction described by the principal technology reference.
