Cyanation of gold ores and concentrates

Globally, the grades of copper, silver and gold ore bodies have been declining, and the mineralogy surrounding them has become more diverse and complex. The cyanidation process for gold production has been dominant for over 130 years due to its selectivity and feasibility in the mining industry. Consequently, the industry has been adapting its methods of extracting gold by utilizing more efficient processes and technologies.

Typically, gold is found in ores and concentrates combined with copper and silver. Therefore, applying cyanide to these types of ores may encounter some difficulties, as the diversity of minerals found in these ores can complicate the application of cyanide.

The cyanation process has become one of the most common methods for recovering gold from ores. Cyanide leaching for gold recovery is based on the properties of gold, so gold will not be oxidized at room temperature. Also, gold is insoluble in sulfuric acid, hydrochloric acid or nitric acid, but it is soluble in aqua regia (a mixture of nitric and hydrochloric acid).

On the other hand, in this case, the most crucial fact about gold is that it is soluble in dilute cyanide solutions. Therefore, in the leaching process, cyanide is used as a leaching agent for gold extraction by this hydrometallurgical method. The hydrometallurgical process begins with a leaching agitator, where the slurry is brought into contact with cyanide, oxygen, water and lime, thereby realizing the leaching process. During leaching, other cyanide complexes such as copper and silver sulfide minerals are formed. During the cyanation stage, the following cyanide complexes are formed. The chemical reaction is as follows:

• Gold cyanide reaction:

4Au + 8NaCN + 2H 2 O + O 2 → 4NaAu(CN) 2 + 4NaOH

(1)

• Silver cyanide reaction:

4Ag + 8NaCN + 2H 2 O + O 2 → 4NaAg(CN) 2 + 4NaOH

(2)

In this case, copper sulfide minerals can form complexes with cyanides such as Cu(CN)2, as shown in the following reaction:

• Copper cyanide reaction:

4Cu + 8NaCN + 2H 2 O + O 2 → 4NaCu(CN) 2 + 4NaOH

(3)

The formation of copper cyanide and silver cyanide complexes affects the cyanide leaching process and gold recovery in the purification and refining stages. These effects mainly interfere with the gold cyanide reaction and carbon adsorption. Most copper minerals react rapidly with cyanide to form various cyanide complexes. It can be seen that chalcopyrite is the copper mineral with the lowest percentage of dissolved and extracted copper compared to other minerals. The carbon management and gold nugget refining costs associated with interference from these cyanide complexes are high. The pH of the cyanide solution plays an important role during the leaching process. The range of cyano complexes depends on conditions such as cyanide concentration and pH.

In the first stage of the leaching process, the formation of copper cyanide and silver cyanide complexes affects the cyanide consumption for gold recovery. Consequently, the operating costs of cyanation increase significantly. As an approximation of cyanide, for every $3,600/tonne delivered to the mine, only 0.28kg/tonne of ore is consumed equal to $1/tonne ore or 1% of gold recovery. Cyanide consumption remains one of the main economic considerations.

The behaviour of a particular ore or concentrate in a test can be determined by performing a bottling test, or by performing a test in a stirred vessel to measure cyanide consumption per unit weight of ore. This value can be scaled up for engineering purposes. The following factors affect the consumption of cyanide during leaching: cyanide concentration; and reaction kinetics.