Uso de acetato de Pb en el Merril Crowe

The beneficial effect of lead ion son precipitation from dilute gold solutions at low temperature is well known. Pb2+ species are reduced at zinc surface, forming cathodically charged areas ofmetallic lead. The negatively charged gold cyanide species are reduced preferentially at these polarized regions. This helps to localize gold deposition, preventing the entire surface of the zinc frombecoming coated with gold and allowing continued corrosion of zinc, which is coupled with further gold deposition. Lead also has a very overpotential for hydrogen evolution and, when deposited on the zincsurface, greatly reduce hydrogen evolution, thereby improving efficiency.
The lead addition rate is critical and is most effective at a Pb-Zn weight ratio of between 1:7 and 1:10. In practice, Pb2+concentration between 0.001 and 0.01 g/L have been found to be optimal for Merrill Crowe precipitation from solutions containing 1 to 10 g/m3 Au. Excess addition is detrimental and can completelystop precipitation, probably due to complete coating zinc particles by a lead film. This effect typically occurs at concentration above 0.06 to 0.10 g/L Pb 2+, but concentration as low as 0.01 g/L may bedetrimental.
Lead has also been shown to reduce the inhibiting effect of zinc hydroxide formation on the zinc surface at low cyanide concentrations.
Other divalent metal cations, such as mercury,thallium, bismuth, cadmium and copper at very low concentrations, have been shown to have similar effects to lead addition to be insignificant.
Another advantage of lead addition is that reacts withsulfide ions in solution, forming insoluble lead sulfide in preference to the highly detrimental sulfide that would otherwise be formed directly on the zinc surface, inhibiting zinc dissolution.Top of Form
El efecto beneficioso de los iones plomo en la precipitación a partir de soluciones diluidas de oro a baja temperatura es bien conocida. Pb2...