In-situ AFM Observation And Mechanism Of The Electrochemical Leaching Of Gold By Anaerobic Thiosulfate-Copper(Ⅱ)-Ammonia Solutions

The contradiction between the increasing demand of gold in the fields of catalysis,medicine,electronics and electrical appliances and the slow growth of the gold leaching recovery has becoming increasingly prominent.However,the leaching of gold by cyanide is not only harmful to the environment,but also unable to effectively leach from refractory gold ore,those problems promoting scientific development of the environmental-friendly gold leaching technology by thiosulfate.At present,the problems of the complex reaction process,the unrevealed detailed industrial mechanism,and the gold dissolution with passivation sulfur film in gold leaching by thiosulfate limit its application in a large scale.In this paper,under the anaerobic conditions,the mesoscopic gold dissolution of Au（111）plane was tracked by combining the electrochemical method with in-situ Electrochemical Atomic Force Microscope（EC-AFM）by changing the applied potential as potentiostatic,the concentration of copper（II）cations and the concentration of ammonia in solutions.The mechanism of gold dissolution with the sulfur passivation on Au（111）plane was discussed.In the system of gold leaching by thiosulfate-copper（II）-ammonia,as increasing the applied potential as potentiostatic from OCP to 0.285 V（vs.SHE）,the dissolution of Au（111）plane changes from the growth of the number of pits,to the layered dissolution mode induced by the increase of the size of pits.The results of the effect of applied potential on the dissolution of Au（111）plane indicating that lower potential value（0.20,0.21 and 0.235（vs.SHE））is liable the pit-number oscillations-induced layer dissolution of Au（111）plane.As the applied potential potentiostatic at 0.235V（vs.SHE）,keeping other components of thiosulfate solution unchanged and increasing the concentration of Cu²⁺（0.25 m M,0.75 m M,1.0 m M,2.0 m M,and 3.0 m M）,the dissolution of Au（111）plane changes from pit-induced layer dissolution to autocatalytic increase of pit-number induced pitting process.As the applied potential potentiostatic at 0.26V（vs.SHE）,keeping other components of thiosulfate solution unchanged and increasing the concentration of ammonia,the dissolution of Au（111）plane presents opposite results compare with the results of changing[Cu²⁺],i.e.,higher ratio of[NH₃]/[Cu²⁺]is liable the parallel direction dissolution of Au（111）plane.However,lower ratio of[NH₃]/[Cu²⁺]is liable the depth increase direction dissolution of Au（111）plane.Based on the above experimental results and analogy to the metastable pitting of steel,mechanism of gold dissolution in copper（II）-ammoniacal-thiosulfate solutions containing the eliminate of passivated sulfur by copper（II）-ammonia complex.The experimental results of the autocatalytic growth of pit-number,oscillatory pit-number accompanied by layered dissolution and the role of each component were discussed.The dissertation has 28 pictures,2 tables,and 113 references.