Technology for Extracting Precious Metals From Refractory Ores

Extra-fine pulverization is one way to extract gold from refractory ores.

Usually, cyanide leaching or gravity concentration processes are sufficient to get ores to yield gold. However, if an ore is refractory---which means the gold is atomically bound to other substances such as carbon, sulfur, copper, antimony or arsenic---other steps may be required.

Autoclave

In the case of sulfide ores, the ore often cannot be ground fine enough to expose the gold to be extracted by cyanide leaching. In this case, high-pressure "cooking" or oxidation of the product, known as autoclaving, can help free up gold so that it can be more directly exposed to the cyanide.

Roasting

With roasting, sulfides are "burned" to extricate them from the gold. However, this process can be environmentally hazardous and costly as the exhaust must subsequently be processed, ultimately becoming more expensive than the roasting itself.

Bio-leach

Bio-leaching harnesses the power of bacteria to break down iron sulfides. After a few hours submerged in a bio-leach solution, the ground ore may then be ready for normal processing.

Chlorination

Chlorination may be used for carbonaceous ores containing gold to deactivate the carbon that would otherwise quickly adsorb the gold dissolved by cyanide.

Extra-fine Grind

Although an expensive solution, grinding down the refractory ore to a superfine consistency may help eliminate additional processing by rendering gold more readily extractable via the usual methods.

References

• Inside Metals: Mining and Processing Basics
• World Gold Council: Extraction
• Denver Mineral Engineers, Inc: The Basic Processes of Gold Recovery