Carbon in pulp
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Introduced in the early 1980s, Carbon in Pulp is regarded as a simple and cheap process. As such it is used in most industrial applications where the presence of competing silver or copper does not prohibit its use.
Activated carbon acts like a sponge to aurocyanide and other complex ions in solution. Hard carbon particles (much larger than the ore particle sizes) can be mixed with the ore and cyanide solution mixture. The gold cyanide complex is adsorbed onto the carbon until it comes to an equilibrium with the gold in solution. Because the carbon particles are much larger than the ore particles, the coarse carbon can then be separated from the slurry by screening using a wire mesh.
Loading gold into carbon
Leached pulp and carbon are transferred in a counter current flow arrangement between a series of tanks, usually numbering 4 to 6. In the final tank, fresh or barren carbon is put in contact with low grade or tailings solution. At this tank the fresh carbon has a high activity and can remove trace amounts of gold (to levels below 0.01 mg/L Au in solution).
As it moves up the train, the carbon loads to higher and higher concentrations of gold, as it comes in contact with higher grade solutions. Typically concentrations as high as 4000 to 8000 grams of gold per tonne of carbon (g/t Au) can be achieved on the final loaded carbon, as it comes in contact with freshly leached ore and pregnant leach solution (PLS). This can be measured by comparing the amount of gold extracted from the carbon to the amount of carbon used.
Removal of gold from carbon
The final loaded carbon then is removed and washed before undergoing "elution" or desorption of gold cyanide at high temperature and pH. Ethanol can also be used to desorb gold from activated carbon, but safety concerns have slowed its acceptance in the gold extraction industry.
The elute solution, normally consisting of caustic soda (the electrolyte), cyanide and water circulates through the loaded carbon extracting gold and other metals. The loaded solution passes through an electro winning cell where the gold and other metals are attached to cathodes, made up of wire wool, by electrolysis. The solution then passes back through the loaded carbon extracting more gold and other metals. This process continues until the carbon has been stripped.
The cathodes (wire wool now plated with gold and other metals) are removed and placed in acid. The acid burns off the wire wool and other metals such as copper and leaves a sediment of gold and solution of acid and dissolved silver. The acid and silver are drained off after which the gold sediment is numerously washed with water.
After the water washes are complete the gold is dried mixed with borax and smelted.
Other gold cyanidation techniques:
- "Dictionary of Mining, Mineral, and Related Terms". Hacettepe University. Archived from the original on 9 June 2011. Retrieved 10 September 2010.