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How are cyanide complexes used to separate gold from its ores?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

The extraction of gold from its ores relies on the formation of extremely stable cyanide complexes, specifically the dicyanoaurate(I) ion, ${Au(CN)}_{2}^{-}$ , which allows the metal to be dissolved into an aqueous solution.12

This process, known as cyanidation, involves the following key steps:

Dissolution: Finely ground ore is treated with a dilute alkaline solution of sodium or potassium cyanide. In the presence of atmospheric oxygen, metallic gold is oxidized and dissolves to form the stable dicyanoaurate complex.34
Separation: Once the gold is dissolved, the gold-bearing solution is separated from the remaining solid ore. This is achieved through methods such as vat leaching for high-grade ores or heap leaching for low-grade ores.4
Recovery: The gold is recovered from the solution using several techniques:
- Adsorption: Granular activated carbon is added to the slurry to adsorb the dissolved gold, after which the carbon is separated and the gold is leached off for further processing.4
- Precipitation: The gold can be recovered via the Merrill-Crowe process, where the solution is deoxygenated and the gold is displaced by reduction with zinc metal powder.34
- Electrowinning: The gold is deposited directly onto steel wool through an electrolytic process.4