Adsorption in pulp is only half of a gold recovery circuit. Once the carbon has taken up the gold cyanide complex, the metal still has to be brought back out and the carbon still has to be made fit for the next pass. Two steps do that work, and they are frequently confused. Elution, or stripping, is the removal of the adsorbed gold from the carbon into a solution that goes on to electrowinning or refining. Reactivation is the thermal treatment that restores the pore structure afterwards. A plant can run an excellent adsorption section and still lose money on a slow elution circuit or a kiln that quietly grinds its carbon away.
Elution works by reversing the adsorption equilibrium, so it is driven by high pH, elevated temperature and a favourable concentration gradient. Plant practice is specific about the conditions. In one documented operation, acid-washed carbon was eluted by pumping a 2% NaCN and 1% Na2CO3 solution through the bed at 120 °C and 125 kPa gauge. The classical high-temperature pressure route is slow: cycles of 16 to 24 hours are typical, which is why plants pay for it in inventory and in carbon sitting idle. Other eluants desorb far faster. Sodium sulphide solutions have been shown to complete elution in about 4 hours at ambient temperature, and a US patent describes more than 99% gold desorption in 5 hours at 80 °C using 20 bed volumes of a methanol-water mixture containing 10 g/L NaOH. Across published desorption studies, temperature is repeatedly identified as the single most important factor.
Between cycles the carbon is acid-washed to dissolve inorganic scale, then thermally reactivated. In a gold plant that normally means a rotary kiln at around 650 °C in a steam atmosphere, which burns off organic foulants and reopens blocked pores. Done properly, the treatment genuinely restores the material: a new-generation reactivation kiln can bring heavily fouled carbon from a flotation-tails carbon-in-leach circuit back to performance comparable with virgin carbon. Under-running or skipping the kiln is one of the quiet ways a plant loses capacity, because fouled carbon still adsorbs — just less. The acid wash is not optional housekeeping either. Loaded carbon carries inorganic scale deposited from the slurry, and that scale blocks pores and consumes capacity that the kiln alone cannot restore, because thermal treatment removes organic foulants rather than mineral deposits. The kiln is also a batch step that often limits how much carbon stays in circulation.
Every pass through that loop costs carbon. Kiln suppliers quote attrition of roughly 5% to 10% per regeneration cycle, and a study of attrition during regeneration reports that gold mines typically lose about 64 grams of activated carbon per tonne of ore processed in the form of fines. That figure is what connects carbon specification to plant economics. Hardness and attrition resistance determine how much of the bed survives each elution and reactivation cycle, and therefore how much new carbon has to be bought every year. A plant that specifies on price per tonne alone is really choosing the rate at which it will repurchase. Carbon consumption in grams per tonne of ore is a better operating indicator than an annual purchase figure: it captures elution losses, kiln attrition and fines washed out at the interstage screens in one number, and it moves as soon as the carbon grade changes.
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