Sodium cyanide is used in the extraction of gold from ore through a process called gold cyanidation, also known as the cyanide process or cyanide leaching. Here’s how it works:
1. Ore preparation: Gold-bearing ore is crushed and ground into a fine powder to increase the surface area for better extraction efficiency.
2. Leaching: The finely ground ore is mixed with a sodium cyanide solution in the presence of oxygen. The cyanide solution selectively leaches gold from the ore, forming a gold-cyanide complex through the following reaction:
4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH
3. Adsorption: The gold-cyanide complex is then adsorbed onto activated carbon, which is added to the leaching tanks. The carbon selectively adsorbs the gold-cyanide complex, separating it from the leached ore.
4. Elution and electrowinning: The loaded carbon is removed from the leaching tanks and the gold is stripped from the carbon using a hot caustic cyanide solution in a process called elution. The gold-rich eluate is then subjected to electrowinning, where an electric current is applied to reduce the gold-cyanide complex, depositing metallic gold onto cathodes.
5. Smelting: The gold recovered from electrowinning is then smelted and refined to obtain high-purity gold bullion.
The cyanide leaching process is highly effective for extracting gold from low-grade ores and has been widely used in the gold mining industry since the late 19th century. However, due to the toxicity of cyanide and its potential environmental impacts, strict regulations and safety measures are in place to minimize risks and ensure proper handling, storage, and disposal of cyanide-containing materials.