Pitting Oscillations Induced Periodic Layer Electrodissolution Of Au（111） And Reaction-Parameter Effects In Cu（Ⅱ）-S₂O₃^2- Alkaline Medium

Leaching of gold by thiosulfate with the advantage of environmentally friendly,low cost,and effective on refractory gold ores,was considered the most potent alternative to the cyanidation method.However,the high consumption of thiosulfate,the sulfur passivation on the gold surface,the unclear mechanism of gold dissolution and the difficult recovery of gold in the leaching solution make it still not applied on a large scale.Therefore,it is very important to reveal the fundamental understanding of gold dissolution at the atomic level for the industrial realization of gold leaching by thiosulfate.In this thesis,in situ electrochemical atomic force microscopy（EC-AFM）was used to monitor the pitting oscilations induced periodic layer dissolution of Au（111）plane in Cu²⁺-NH₃-S₂O₃^2-solution,to explore the mechanism of electrochemical leaching of gold by thiosulfate and the role of each component,together with the theoretical basis of gold dissolution kinetics,and to find the control method of gold dissolution reaction process.Analysis of the CVs of the dissolution of gold by thiosulfate to determines the applied potential for gold dissolution.According to the effect of substances in the Cu²⁺-NH₃-S₂O₃^2-system and the interfacial electrochemical reaction,the cyclic voltammetry scanning of Au（111）spherical electrodes in thiosulfate solution with different p H and concentration ratio of cupric-ammonia solution was performed,analyzing the kinetics process of electrochemical reactions on the gold surface,ascertain the CV peaks of Cu⁺/Cu⁰,Cu²⁺/Cu⁺,Cu（I）/Cu⁰,and Cu（Ⅱ）/Cu（Ⅰ）,as well as the oxidation peak of S₂O₃^2-.The potential of gold dissolution was confirmed from 0.190 V（SHE）.Atomic force microscope（AFM）was used to track the crystal surface morphology of gold dissolved by thiosulfate to determine the general dissolution process and the effect of cupric-ammonia of the single crystal surface.The effect of applied voltage on the rate of gold dissolution by thiosulfate was investigated,and the applied potential of gold dissolution reaction in the Cu²⁺-NH₃-S₂O₃^2-solution was controlled between 0.190V and 0.22 V（SHE）.It was found that the higher the applied potential,the faster rate of the gold leaching.As the applied potential（0.21V,0.22V（SHE））is higher than the open circuit potential（0.20V（SHE））,the terrace surface dissolves rapidly,the growth of the pits area to the disappearance of the terrace surface unable to be traced by EC-AFM in real-time.However,as the applied potential set at 0.190 V,0.195 V,0.198V（SHE）(E<E_OCP),the dissolution rate of the Au（111）surface slowed down,and the appearance,growth,and dissolution process of Au（111）surface could be monitored by EC-AFM,where the pits appear randomly on the terrace,and new pits appear around it later,leading to intensified dissolution.The pits gradually increase larger,and adjacent pits continue to grow and merge together,completing the dissolution of a whole layer of Au（111）surface.In view of the complexity of the cupric-ammonia mechanism in the process of gold dissolution by thiosulfate,the effect of the ratio of copper-ammonia concentration on the electrolysis of Au（111）in Cu²⁺-NH₃-S₂O₃^2-solution was studied,which indicate that Au（111）plane was first to eliminate the passivation film aiming at dissolving the exposed Au atoms.Instead of the dissolution of gold through the direct oxidation by cupric anions as assumed in previous studies,the increase of the cupric-ammonia concentration ratio is beneficial to the number of the pits of autocatalytic pitting dissolution.The reduction of the cupric-ammonia concentration ratio leads to periodic dissolutions with oscillations in the number and area of pits on the Au（111）surface.The three-step cyclic reaction mechanism of Au（111）dissolution in an anaerobic S₂O₃^2-solution of copper ammonia complex was proposed based on the experimental results.After a series of thiosulfate reactions,the formation of elemental sulfur is deposited on the surface of Au（111）,which inhibits the dissolution of gold.The cupric-ammonia complex dissolves the passivated elemental sulfur to form Cu（Ⅰ）,then the bare Au atoms are dissolved and coordinated with S₂O₃^2-anions,Cu（Ⅰ）can be oxidized to Cu（Ⅱ）electrochemically and continues the reaction cycle.