Absorption with Activated Carbon

 To solve the problems presented by the classic scheme of cyanidation, the absorption with activated carbon was developed, which also dates back to the past century, which in its beginnings was of very restricted use, due to the fact that the recuperation of precious metals absorbed in the carbon had to be performed by flotation or by baking and burning. In the present, there are various alternatives which exist for desorbing the precious metals and to be able to pass them previously to electrodepositation. In this process columns of absorption are used, in which the the contact is performed in countercurrent in variousreservoirs in series in which the solution advances from down to up in a compatible form with the velocity of sedimentation and granulometry of the carbon. Other more recent equipments of design are the columns of fluidized beds, as is the case of the Nimcix developed by the Angloamerican.

Finally, we can mention the system used in Cripple Creek, United States, which consists in a species of special containers which contain the carbon, which is fluidized by the solution (5).once the conjunction of containers is loaded it is transferred to the section of elusion in which they fulfill the role of reactors of elusion. These containers are of steel covered with a special plastic for resisting acid washings, if it is necessary. This system is also used in Australia and in Zimbabwe. One recent innovation was installed in Whim Creek Consolidated, United States, in which the containers instead of piling in the form of columns are disposed in a carroussel.

 The elusion of carbon can be effectuated through hot solutions of NaOH and NaCN at atmospheric pressure, which was the first in being used successfully, or the use of pressurized reactors for working with this same type of solutions from above the point of boiling and the use ofethanol or glycol for working under the point of boiling (6). In all these alternatives the carbon can be regenerated and reused many times.

The electrodepositation is performed fundamentally in cylindric cells Zadra type, from which different designs exist. In the present time, rectangular cells are preferred, for the reason of rationalization of the space and its handling, but what is essential still remains the same: electrolite of NaOH – NaCN and cathodes of wool of steel.

Curiously, the use of the activated carbon has meant a higher use of the precipitation with Zinc, but now starting with the eluision. The first company in using it in the Untied States was the Freeport McMorand Gold Inc., and posteriorly, it was introduced in Equity Silver, Canada.