The effect of cyanide and lead ions on the cementation rate, stoichiometry and morphology of silver in cementation from cyanide solutions with zinc powder

The investigation explores the cementation of silver using zinc powder from cyanide solutions that vary in concentration, both in the presence and absence of lead ions. Utilizing stirred reactor batch tests and scanning electron microscopy, the study identifies how cyanide ion concentration affects the morphology of the cementation product, the kinetics of the reaction, and the nature of the cementation process. Three distinct concentration regimes of cyanide are established: a low concentration regime characterized by an ion-exchange reaction at the interface of zinc and the solution leading to uniform silver growth; a high concentration regime that reflects industrial practices where the chemical reaction involves noticeable hydrogen evolution and a one-to-one molar ratio of silver to zinc; and an intermediate regime serving as a transition state. The results indicate that in low and intermediate regimes, lead and cyanide ions do not significantly alter silver morphology but enhance cementation kinetics due to Zn(OH)₂ instability. Conversely, in high cyanide concentrations, lead ions modify the silver deposit from dense-branching to dendritic morphology without significantly impacting kinetics. The findings underscore the complex interactions between cyanide and lead ions in silver cementation processes.