As a result of developments in the past 40 years, the carbon-in-pulp (CIP) process has become the preferred route to the recovery of gold from low-grade cyanidation pulps. The use of columns of activated carbon for the recovery of gold and silver from mine waste water and from the clarified solutions resulting from dump leaching is also becoming an established process. In these processes, the most important operation is that of the absorption circuit, since the efficiency of the gold recovery by activated carbon determines the overall recovery rate of the plant, as well as the size of the elution, reactivation, and electro-winning circuits.

Over several years, an empirical but simple approach to the modeling of the absorption process has evolved at the Council for Mineral Technology (Mintek). This approach is being used effectively in the design and optimization of a number of local CIP plants. The basis of the Mintek approach is the realization that the kinetics of the absorption process are the most important factors governing the performance of the absorption circuit. Fig. 1 serves to demonstrate this point: equilibrium between the gold on carbon and that in solution was not reached even after a period of 300 hours when the carbon was isolated in a well-mixed basket in one stage of a continuous CIP plant. The mean residence time of the carbon seldom exceeds 48 hours per stage in an operating plant, so that true equilibrium is never achieved, and any rational approach to the modeling...

see the paper attached here.

And The Chemistry of Gold Extraction By John Marsden, Iain House