EXTRACTION OF GOLD
Gold occurs principally as a native metal, usually alloyed to a greater or lesser extent with silver or sometimes with mercury ( amalgam). Native gold can occur as sizeable nuggets, as fine grains or flakes in alluvial deposits, or as grains or microscopic particles embedded in other rocks. Gravity concentration has been historically the most important way of extracting the native metal using pans or washing tables. However, froth flotation processes may also be used to concentrate the gold. Froth flotation is usually applied when the gold present in an ore is closely associated with sulfide minerals such as pyrite. In this case, concentration of the sulfides results in concentration of gold values. Generally, recovery of the gold from the sulfide concentrates requires further processing, usually by roasting or wet pressure oxidation. If the gold cannot be concentrated for smelting, then it is leached by an aqueous solution:
- The cyanide process is the industry standard.
- Thiosulfate leaching has been proven to be effective on ores with high soluble copper values
A "refractory" gold ore is an ore that is naturally resistant to recovery by standard cyanidation and carbon adsorption processes. These refractory ores require pre-treatment in order for cyanidation to be effective in recovery of the gold. A refractory ore generally contains sulfide minerals, organic carbon, or both .Pre-treatment options for refractory ores include:
- Roasting
- Bio-oxidation
- Pressure oxidation
- Ultrafine grinding
