A study of the gold colloid dissolution kinetics in oxygenated ammoniacal thiosulfate solutions

摘要

Colloidal gold has a characteristic absorption peak at 530 nm that can be used to monitor the extent of gold dissolution in different lixiviant systems using an ultraviolet-visible spectrophotometer. This paper reports the kinetics of the dissolution of gold colloid in ammoniacal thiosulfate solutions using oxygen as a oxidant at pH 9.3-9.5 and temperature 25-48 deg C. The relative rates of gold dissolution in different lixiviant systems are in descending order: oxygen-cyanide > copper(II)-ammonia-thiosulfate > oxygen-ammonia-thiosulfate > oxygen-ammonia > copper(II)-ammonia. The predominant gold(I) species in oxygenated thiosulfate solution based on the measured potentials on gold electrode is gold-thiosulfate complex (Au(S_2O_3)~(3-)_2). However, the measured potentials in the absence of thiosulfate are close to the reported values for the Au(NH_3)~(3+)_4 /Au(NH_3)~+_2 couple, which indicates that disproportionation or oxidation of Au(NH_3)~+_2 occurs. The initial rates of gold dissolution show reaction orders of approx = 0.3 with respect to oxygen and ammonia, and are independent of the concentration of thiosulfate and chloride. The dissolution kinetics follow a shrinking core model with an activation energy of 25-37 kJ mol~(-1), indicating a mixed chemical-diffusion-controlled reaction.