CARBON IN PULP PROCESS
The most cost-effective process is to create absorption of the dissolved gold onto activated carbon, resulting in an easier solid-solid separation based on size. To achieve this, the ore particles must typically be smaller than 100 µm while the carbon particles must be larger than 14 mesh. Absorption is achieved by contacting the activated carbon with the agitated pulp. This can be done while the gold is still being leached with the CIL-process, or following leaching with the CIP-process. The CIL-process offers the advantage of countering the adsorption of gold on carbonaceous or shale ore particles, but is more expensive due to less efficient adsorption, increased gold inventory and increased fouling and abrasion of the carbon.
Activated carbon in contact with a pulp containing gold can typically recover more than 99.5% of the gold in the solution in 8 to 24 hours, depending on the reactivity of the carbon, the amount of carbon used and the mixer's efficiency. The loaded carbon is then separated from the pulp by screens that are air or hydrodynamically swept, thus preventing blinding by the near sized carbon particles. The pulp residue is then either thickened to separate the cyanide containing solution for recovery/destruction of the cyanide, or sent directly to the tailings storage facility from which the cyanide containing solution is recycled to the leach plant.
The gold adsorbed on the activated carbon is recovered from the carbon by elution, typically with a hot caustic aqueous cyanide solution. The carbon is then regenerated and returned to the adsorption circuit while the gold is recovered from the eluate using either zinc cementation or electrowinning. If it contains significant amounts of base metals, the gold concentrate is then either calcined or directly smelted and refined to gold bullion that typically contains about 70 - 90% gold. The bullion is then further refined to either 99.99% or 99.999% fineness using chlorination, smelting and electro-refining. High purity gold is taken directly from activated carbon eluates, using recently developed processes that utilize solvent extraction to produce intensive leaching of gravity concentrate. See Fig. 3