The effect of silver on the acidic ferric sulfate leaching of primary copper sulfides under recycle solution conditions observed in heap leaching. Part 1: Kinetics and reaction mechanisms

The current focus for the treatment of low-grade copper resources is through heap leaching with the aid of bacteria. However, it is well known that chalcopyrite and other copper sulfide minerals such as enargite are refractory and suffer from slow leaching kinetics and poor overall extraction due to becoming passivated. Researchers have observed that certain additives such as silver can considerably improve the extraction. What is less understood is if additives like silver can effectively promote the extraction under recycle heap leach conditions, and if so, the mechanism of its action. The study reported in this paper addresses these questions. The standard leaching conditions used were based on a simulated long term recycle solution composition. It was observed that silver addition greatly promoted the extraction of copper from the ore samples under those conditions. Experiments at varied silver concentrations showed that there is a minimum concentration of silver needed to achieve a significant enhancement in the rate and extent of the extraction of copper. For example, for the leaching of the chalcopyrite ore tested in the present study a silver addition of 0.02 g/L resulted in 25% copper extraction in 10 days, whereas 0.05 g/L Ag resulted in over 90% extraction in the same period. It has also been shown that the silver needs to be present at the start of the leaching or precipitated onto the surface of the minerals before adding the lixiviant, to achieve the catalytic effect. Iron extraction appeared to be retarded in the presence of silver, which suggests that silver not only catalyses chalcopyrite and enargite leaching but it may also be galvanically protecting pyrite from dissolution, although the lower iron observed is at least partly attributable to the formation of argentojarosite.