The cementation of gold by iron from cyanide solutions

Abstract

The cementation of gold by iron from cyanide solutions was studied using electrochemical and chemical techniques. From a superposition of the polarization curves for iron oxidation and Au((CN) 2 − reduction (pH range: 9–14, free CN − concentration: (0–10 −1 M), it was concluded that gold cementation was most likely to occur at pH 9 in solutions containing little free cyanide and at pH 14 in solutions containing high levels of cyanide. The reactions were also predicted to be under chemical control with a high activation energy. A qualitative study of gold cementation by iron over the pH and the free CN − concentration ranges of interest confirmed that the amount of gold deposited in a given time is greatest in heated solutions, either when the pH is 9 and there is no added cyanide, or when the pH is 14 and the cyanide concentration is high (say 1 M or more). Quantitative measurements of the cementation rate at pH 9 with no added cyanide were made by dissolving the gold deposited and analysing the resulting solution for gold using atomic absorption spectrophotometry. It was shown that the reaction rate was first order in Au(CN) 2 − in the initial states, the rate constant at 25°C being (0.35 ± 0.05) × 10 −6 m s −1 and the activation energy 50 ± 5 kJ mol −1 between 25°C and 84°C i.e. the reaction is chemically controlled.