Activated carbon adsorbs gold (as gold-cyanide complex) from pregnant leach solution onto its high-surface-area pore structure. Loaded carbon can then be desorbed (stripped) to release the gold back into a small volume of solution for electrowinning. The mechanism is physical adsorption combined with weak chemisorption — fully reversible by heating with caustic.
Why Activated Carbon
Activated carbon has an enormous internal surface area (~1,000 m²/g) of pores in the 1–10 nm range. The Au(CN)₂⁻ complex fits into these pores and is held by van der Waals forces plus weak interactions with surface functional groups. The result: carbon can load up to 10,000 g Au per tonne of carbon — concentrating dilute leach solution by a factor of 1,000 to 10,000.
Loading Rate
Loading rate depends on solution gold tenor, contact time, agitation, and carbon activity (freshness). Typical equilibrium loadings: 5,000–8,000 g/t in modern operations. Working loadings (carbon transferred to strip) are usually 80–90% of equilibrium.
Carbon Selectivity
Activated carbon strongly prefers gold over silver, copper, zinc, mercury, and base metals. This is why silver-dominant solutions go to Merrill-Crowe instead — carbon adsorption would leave most of the silver in tailings. For gold-dominant solutions, carbon recovers 99%+ of the dissolved gold.
Regeneration
After stripping (Zadra or AARL), carbon is acid-washed with HCl to remove calcium carbonate and base-metal contaminants. Eventually thermal regeneration in a rotary kiln (650–750°C) is needed to restore loading capacity. KCA has delivered carbon regeneration units at 25 and 75 kg/h scales — and the patented KCA Carbon Converter for converting depleted fines to recoverable ash.
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