An electrochemical study of silver recovery in thiosulfate solutions. A window towards the development of a simultaneous electroleaching-electrodeposition process

The simultaneous electroleaching-electrodeposition of silver was studied systematically to develop an alternate process for the recovery of metallic silver. The system was assessed employing electrochemical and microstructural characterization techniques in a thiosulfate-ammonia medium. The electrochemical studies revealed the dependence of silver electroleaching rate in thiosulfate-ammonia solutions on different processes controlled by anodic potential. At the anodic potential below 200 mVSHE, the silver is electro-oxidized and complexed by the applied potential and thiosulfate, respectively. Increasing the potential up to 445 mVSHE, the thiosulfate is oxidized and the silver leaching current is decreased due to the diffusional control of the leaching. It was also found that the sulfite species formed from the thiosulfate decomposition can leach small amounts of metallic silver. Finally, at potentials from 695 to 795 mVSHE, the thiosulfate is further decomposed and a silver sulfide film is generated which inhibits the silver dissolution. The simultaneous electroleaching-electrodeposition of silver was also studied in an electrolytic cell with graphite electrodes at room temperature. The results showed that a 74% of silver can be recovered at a low applied current density of 0.066 mA/cm2 with a negligible thiosulfate decomposition and a current efficiency of 76.54%. On the other hand, when the current is increased the silver recovery and current efficiency is decreased, due to the oxidative decomposition of thiosulfate which promotes the formation of silver sulfide. These results are corroborated by the X-ray diffraction and SEM-EDS analyses.