Gold Extraction Methods
Gold has been known and highly valued since prehistoric times. It may have been the first metal used by humans and was valued for ornamentation and rituals. Approximately 60% of the gold mined today is held by government and central banks and gold is presently the most significant means of international payment.
Gold Minerals
In nature, gold most often occurs in its native state (that is, as a metal), though usually alloyed with silver. Native gold contains usually eight to ten percent silver, but often much more — alloys with a silver content over 20% are called electrum.
The average concentration of gold in the world is about 0.005 g/t which is lower than other metals. The low concentration of gold in primary rocks means that upgrading by a factor of 3000-4000 is usually required during ore formation processes to achieve commercial concentrations. This may be possible using natural gravity concentration processes or leaching gold with natural fluids from the host rock. Thus, by highly oxidizing, acidic and complexing (chloride) solutions, followed by redeposition in a more concentrate form. Owing to its siderophile properties (weak affinity for oxygen and sulfur, high affinity for metals) gold tends to concentrate in residual hydrothermal fluids and subsequent metallic or sulphidic phases, rather than silicates, which form at an early stage of magma cooling. Rocks that are high in clays and low in carbonates are the best sources of gold, and reprecipitation occurs when the hydrothermal solutions encounter a reducing environment, such as a region of high carbonate, carbon or reducing sulphide contain.
Gold Extraction Methods
Although new process are being proposed on a regular basis, there have in fact been no dramatic changes in the metallurgical techniques for gold extraction since the introduction of the cyanide process (cyanide leaching or cyanidation) by McArthur and Forrester in 1887. A basic flowchart for the recovery of gold from its ore is provided in Figure 1.
The major categories of commercially viable recovery processes include the following:
1. Amalgamation (with mercury)
2. Gravity Concentration (using jogs, tables, spirals, Reichert cone, moving belt separator, etc.)
3. Flotation (as free particles or contained in base metal sulfide concentrates)
4. Pyrometallurgy (in the smelting and refining of base metal ores and concentrates)
5. Hydrometallurgy (direct cyanidation, cyanidation with carbon adsorption, heap-leach and chlorination-leach)
6. Refractory ore processing
7. Alternative lixiviants
Last update: February 15, 2012
