Recovery of gold by electrowinning from activated carbon stripping eluates is an important part of the gold recovery process developed by Zadra and was implemented in Carbon-in-Pulp and Carbon-in-Leach, and Heap Leaching processes. The electrolytic process has replaced the Merrill-Crowe process in some gold operations where the gold content is relatively high in the pregnant solution. Basically, electrowinning cells are widely employed to recover gold from eluant produced by desorption from activated carbon. Packed bed cathodes designs are used to treat solution in the range from 4 to 900 ppm gold. Gold may be electrowon from high tenor solutions obtained by reflux solvent elution by simple plate type cells. Essentially, direct electrowinning of gold and silver from heap leach pregnant solutions and the appropriate cell must be selected considering metallurgical and economical aspects.
In most gold operations, current elution practice produces two types of eluates, medium and high grade. The first one contains 4 to 900 ppm gold and the second group contains more than 900 ppm gold. The former need packed bed cathode cells for satisfactory recovery while high gold content solutions can be plated in ordinary plane sheet cells. In many circumstances, metallurgists have not paid much attention to the performance of electrowinning cells in operating gold plants due to it was considered that current efficiency is not important since it is a small proportion of the total operating cost. Other factors are relevant and must be studied because they are related to the overall operation performance. It has been noted that scavenge gold cyanide ions to low levels in plants slurries and solutions requires an activated carbon with high activity and low initial gold loading. Equilibrium adsorption may be explained considering Freundlich equation, C1 = bCn , where C1 is gold in carbon, C is gold in solution and b,n are constants. In practice equilibrium is seldom obtained and kinetics must be considered. In this case, Langmuir Isotherm is important.
Efficient electrowinning is influenced by barren values leaving adsorption and the speed of elution of activated carbon and its residual gold loading. Under theoretical considerations, the gold cyanide remaining on activated carbon, it is reduced to metallic gold above a temperature of 340-380oC in the course of reactivation. This consideration could create a progressive build up of non-recoverable gold values.
Gold electrowinning cell