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 and Recovery Processes

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

Flotation

The flotation process consists of producing a mineral concentrate through the use of chemical conditioning agents followed by intense agitation and air sparging of the agitated ore slurry to produce a mineral rich foam concentrate . The process is said to have been invented by a miner who watched the process happening while washing dirty work clothing in his home washing machine.

Specific chemicals are added to either float (foam off) specific minerals or to depress the flotation of other minerals. Several stages of processing are generally involved with rough bulk flotation products being subjected to additional flotation steps to increase product purity.

The flotation process in general does not float free gold particles but is particularly effective when gold is associated with sulfide minerals such as pyrites. In a typical pyrytic gold ore, the gold is encapsulated within an iron sulfide crystal structure. Highly oxidized ores generally do not respond well to flotation. Advantages of the flotation process are that gold values are generally liberated at a fairly coarse particle size (28 mesh) which means that ore grinding costs are minimized. The reagents used for flotation are generally not toxic, which means that tailings disposal costs are low.

Flotation will frequently be used when gold is recovered in conjunction with other metals such as copper, lead, or zinc. Flotation concentrates are usually sent to an off-site smelting facility for recovery of gold and base metals.

Cyanide leaching is frequently used in conjunction with flotation. Cyanidation of flotation concentrates or flotation tailings is done depending upon the specific mineralogy and flowsheet economics.

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