The cyanidation process for gold extraction is a widely used hydrometallurgical technique that involves dissolving gold from ore using a dilute cyanide solution. This method is highly effective for recovering gold from low-grade ores and has been the industry standard for over a century. Below is a detailed breakdown of the cyanidation process, covering its key stages and operational considerations.
1. Ore Preparation
Before cyanidation can begin, the ore must be properly prepared to ensure optimal contact between the gold particles and the cyanide solution. This typically involves crushing and grinding the ore to a fine consistency, increasing the surface area for chemical reactions. In some cases, additional processes like gravity separation or flotation may be employed to concentrate the gold-bearing material before leaching.
2. Leaching
The core of the cyanidation process is leaching, where finely ground ore is mixed with a weak sodium cyanide (NaCN) solution (usually 0.01% to 0.05% concentration). The slurry is agitated in large tanks or vats to maintain uniform contact between the solution and ore particles. Oxygen is introduced to facilitate the dissolution of gold, as it reacts with cyanide to form a soluble gold-cyanide complex (Au(CN)₂⁻). The chemical reaction can be represented as:
4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH
The leaching process typically takes 24–48 hours, depending on factors such as ore composition, particle size, and cyanide concentration. Temperature and pH levels are carefully controlled (usually maintained at pH 10–11 using lime) to prevent the formation of toxic hydrogen cyanide gas.
3. Solid-Liquid Separation
After leaching, the slurry undergoes solid-liquid separation to isolate the gold-rich solution (pregnant solution) from the residual solids (tailings). This is commonly achieved through filtration or counter-current decantation (CCD), where multiple thickeners are used to wash and separate the solids from the liquid phase.
4. Gold Recovery
The pregnant solution containing dissolved gold is then processed to recover the precious metal. The most common methods include:
- Carbon Adsorption (CIP/CIL): Activated carbon is used to adsorb gold-cyanide complexes from the solution. In Carbon-in-Pulp (CIP), carbon is added after leaching, while Carbon-in-Leach (CIL) integrates carbon addition during leaching. The loaded carbon is