UBC Theses and Dissertations
Fundamental aspects of gold leaching in thiosulfate solutions Li, Cheng
The kinetics and mechanism of gold leaching in the ammonia-thiosulfate-copper (ATS-Cu) system have been studied using the rotating electrochemical quartz crystal microbalance (REQCM). Anodic polarization, cathodic polarization and leaching experiments were included in this study. The effect of concentration of different reagents, applied potential, pH, temperature, and electrode rotating velocity on the gold leaching rate have been investigated. The effect of solution copper species on the anodic reaction of gold dissolution in thiosulfate solution was elucidated using the REQCM. With respect to the role of copper on the anodic processes, two possible mechanisms were proposed. It was shown that, in the absence of S₂O₃²⁻, the electrochemical reaction on the cathode is the reduction of Cu(NH₃)₄²⁺+ to Cu(NH₃)₂⁺ in the potential range of 0.2 to -0.3 V vs. SHE. Based on other experimental results, it is believed that, in the presence of S₂O₃²⁻, the electrochemical reaction on the cathode also is the same as the reaction in the absence of S₂O₃²⁻. It was found that the anodic process and leaching process are under chemical reaction control, but the cathodic process is under diffusion control. It may therefore be concluded that the leaching process is under anodic control under the majority of conditions tested (cathodic control will be important when the cupric ammine species is very dilute in solution). Experiments show that increasing the ratio of NH3/ S₂O₃²⁻ will favor the leaching rate. However, excess thiosulfate or excess ammonia may inhibit the dissolution of gold. The effect of selected additives was also studied.
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