Carbon in pulp

Activated carbon acts like a sponge to gold cyanide and other metal complexes 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 adsorped onto the carbon until it comes to an equilibrium with the gold in solution. The coarse carbon can then be separated from the slurry by screening using a wire mesh. Leached pulp and carbon are transfered 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 in this tank the fresh carbon has a high activity and can remove 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 4000 to 8000 g/t Au concentrations can be achieved on the final loaded carbon, as it comes in contact with freshly leached ore and pregnant leach solution. This loaded carbon then is removed and washed before undergoing "elution" or "desorption" of gold at high cyanide temperature and pH. Ethanol can also be used to desorp gold from activated carbon, but saftey concerns has slowed its acceptance in the industry. The rich eluate sultion that emerges from the elution process is passed through electrowinning cells where gold and other metals are precipitaed on the cathodes. Smelting of the cathodes material further refines the gold and produces gold ingots suitable for transport to the refinery.